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Sleep is a sedentary state of mind and body. It is characterized by altered
consciousness Consciousness, at its simplest, is sentience and awareness of internal and external existence. However, the lack of definitions has led to millennia of analyses, explanations and debates by philosophers, theologians, linguisticians, and scien ...
, relatively inhibited sensory activity, reduced
muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of mus ...
activity and reduced interactions with surroundings. It is distinguished from
wakefulness Wakefulness is a daily recurring brain state and state of consciousness in which an individual is conscious and engages in coherent cognitive and behavioral responses to the external world. Being awake is the opposite of being asleep, in which m ...
by a decreased ability to react to stimuli, but more reactive than a
coma A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to respond normally to painful stimuli, light, or sound, lacks a normal wake-sleep cycle and does not initiate voluntary actions. Coma patients exhi ...
or
disorders of consciousness Disorders of consciousness are medical conditions that inhibit consciousness. Some define disorders of consciousness as any change from complete self-awareness to inhibited or absent self-awareness and arousal. This category generally includes min ...
, with sleep displaying different, active brain patterns. Sleep occurs in repeating periods, in which the body alternates between two distinct modes: REM sleep and
non-REM Non-rapid eye movement sleep (NREM), also known as quiescent sleep, is, collectively, sleep stages 1–3, previously known as stages 1–4. Rapid eye movement sleep (REM) is not included. There are distinct electroencephalographic and other cha ...
sleep. Although REM stands for "rapid eye movement", this mode of sleep has many other aspects, including virtual
paralysis Paralysis (also known as plegia) is a loss of motor function in one or more muscles. Paralysis can also be accompanied by a loss of feeling (sensory loss) in the affected area if there is sensory damage. In the United States, roughly 1 in 5 ...
of the body.
Dreams A dream is a succession of images, ideas, emotions, and sensations that usually occur involuntarily in the mind during certain stages of sleep. Humans spend about two hours dreaming per night, and each dream lasts around 5 to 20 minutes, althou ...
are a succession of images, ideas, emotions, and sensations that usually occur involuntarily in the
mind The mind is the set of faculties responsible for all mental phenomena. Often the term is also identified with the phenomena themselves. These faculties include thought, imagination, memory, will, and sensation. They are responsible for various m ...
during certain stages of sleep. During sleep, most of the body's systems are in an
anabolic Anabolism () is the set of metabolic pathways that construct molecules from smaller units. These reactions require energy, known also as an endergonic process. Anabolism is the building-up aspect of metabolism, whereas catabolism is the breakin ...
state, helping to restore the immune, nervous,
skeletal A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside ...
, and
muscular Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscle ...
systems; these are vital processes that maintain mood, memory, and cognitive function, and play a large role in the function of the
endocrine The endocrine system is a messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating distant target organs. In vertebrates, the hypothalamus is the neur ...
and
immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinte ...
s. The internal
circadian clock A circadian clock, or circadian oscillator, is a biochemical oscillator that cycles with a stable phase and is synchronized with solar time. Such a clock's ''in vivo'' period is necessarily almost exactly 24 hours (the earth's current solar day ...
promotes sleep daily at night. The diverse purposes and mechanisms of sleep are the subject of substantial ongoing research. Sleep is a highly conserved behavior across animal evolution. Humans may suffer from various
sleep disorder A sleep disorder, or somnipathy, is a medical disorder of an individual's sleep patterns. Some sleep disorders are severe enough to interfere with normal physical, mental, social and emotional functioning. Polysomnography and actigraphy are tests ...
s, including
dyssomnia Dyssomnias are a broad classification of sleeping disorders involving difficulty getting to sleep, remaining asleep, or of excessive sleepiness. Dyssomnias are primary disorders of initiating or maintaining sleep or of excessive sleepiness and ar ...
s such as
insomnia Insomnia, also known as sleeplessness, is a sleep disorder in which people have trouble sleeping. They may have difficulty falling asleep, or staying asleep as long as desired. Insomnia is typically followed by daytime sleepiness, low energy, ...
,
hypersomnia Hypersomnia is a neurological disorder of excessive time spent sleeping or excessive sleepiness. It can have many possible causes (such as seasonal affective disorder) and can cause distress and problems with functioning. In the fifth edition ...
,
narcolepsy Narcolepsy is a long-term neurological disorder that involves a decreased ability to regulate sleep–wake cycles. Symptoms often include periods of excessive daytime sleepiness and brief involuntary sleep episodes. About 70% of those affect ...
, and
sleep apnea Sleep apnea, also spelled sleep apnoea, is a sleep disorder in which pauses in breathing or periods of shallow breathing during sleep occur more often than normal. Each pause can last for a few seconds to a few minutes and they happen many tim ...
;
parasomnia Parasomnias are a category of sleep disorders that involve abnormal movements, behaviors, emotions, perceptions, and dreams that occur while falling asleep, sleeping, between sleep stages, or during arousal from sleep. Parasomnias are dissociat ...
s such as
sleepwalking Sleepwalking, also known as somnambulism or noctambulism, is a phenomenon of combined sleep and wakefulness. It is classified as a sleep disorder belonging to the parasomnia family. It occurs during slow wave stage of sleep, in a state of lo ...
and rapid eye movement sleep behavior disorder;
bruxism Bruxism is excessive teeth grinding or jaw clenching. It is an oral parafunctional activity; i.e., it is unrelated to normal function such as eating or talking. Bruxism is a common behavior; reports of prevalence range from 8% to 31% in the gen ...
; and
circadian rhythm sleep disorder Circadian rhythm sleep disorders (CRSD), also known as circadian rhythm sleep-wake disorders (CRSWD), are a family of sleep disorders which affect the timing of sleep. CRSDs arise from a persistent pattern of sleep/wake disturbances that can be ...
s. The use of
artificial light Lighting or illumination is the deliberate use of light to achieve practical or aesthetic effects. Lighting includes the use of both artificial light sources like lamps and light fixtures, as well as natural illumination by capturing daylig ...
has substantially altered humanity's sleep patterns. Common sources of artificial light include the screens of electronic devices such as
smartphone A smartphone is a portable computer device that combines mobile telephone and computing functions into one unit. They are distinguished from feature phones by their stronger hardware capabilities and extensive mobile operating systems, whi ...
s and televisions, which emit large amounts of blue light, a form of light typically associated with daytime. This disrupts the release of the hormone
melatonin Melatonin is a natural product found in plants and animals. It is primarily known in animals as a hormone released by the pineal gland in the brain at night, and has long been associated with control of the sleep–wake cycle. In vertebrat ...
needed to regulate the sleep-cycle.


Physiology

The most pronounced physiological changes in sleep occur in the brain. The brain uses significantly less energy during sleep than it does when awake, especially during non-REM sleep. In areas with reduced activity, the brain restores its supply of
adenosine triphosphate Adenosine triphosphate (ATP) is an organic compound that provides energy to drive many processes in living cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all known forms ...
(ATP), the molecule used for short-term storage and transport of energy. In quiet waking, the brain is responsible for 20% of the body's energy use, thus this reduction has a noticeable effect on overall energy consumption. Sleep increases the
sensory threshold In psychophysics, sensory threshold is the weakest stimulus that an organism can sense. Unless otherwise indicated, it is usually defined as the weakest stimulus that can be detected half the time, for example, as indicated by a point on a proba ...
. In other words, sleeping persons perceive fewer stimuli, but can generally still respond to loud noises and other salient sensory events. During slow-wave sleep, humans secrete bursts of
growth hormone Growth hormone (GH) or somatotropin, also known as human growth hormone (hGH or HGH) in its human form, is a peptide hormone that stimulates growth, cell reproduction, and cell regeneration in humans and other animals. It is thus important in ...
. All sleep, even during the day, is associated with the secretion of
prolactin Prolactin (PRL), also known as lactotropin, is a protein best known for its role in enabling mammals to produce milk. It is influential in over 300 separate processes in various vertebrates, including humans. Prolactin is secreted from the pi ...
. Key physiological methods for monitoring and measuring changes during sleep include
electroencephalography Electroencephalography (EEG) is a method to record an electrogram of the spontaneous electrical activity of the brain. The biosignals detected by EEG have been shown to represent the postsynaptic potentials of pyramidal neurons in the neocorte ...
(EEG) of brain waves,
electrooculography Electrooculography (EOG) is a technique for measuring the corneo-retinal standing potential that exists between the front and the back of the human eye. The resulting signal is called the electrooculogram. Primary applications are in ophthalmol ...
(EOG) of eye movements, and
electromyography Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG is performed using an instrument called an electromyograph to produce a record called an electromyogram. An electromyo ...
(EMG) of
skeletal muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of m ...
activity. Simultaneous collection of these measurements is called
polysomnography Polysomnography (PSG), a type of sleep study, is a multi-parameter study of sleep and a diagnostic tool in sleep medicine. The test result is called a polysomnogram, also abbreviated PSG. The name is derived from Greek and Latin roots: the Gre ...
, and can be performed in a specialized sleep laboratory. Sleep researchers also use simplified
electrocardiography Electrocardiography is the process of producing an electrocardiogram (ECG or EKG), a recording of the heart's electrical activity. It is an electrogram of the heart which is a graph of voltage versus time of the electrical activity of the hear ...
(EKG) for cardiac activity and
actigraphy Actigraphy is a non-invasive method of monitoring human rest/activity cycles. A small actigraph unit, also called an actimetry sensor, is worn for a week or more to measure gross motor activity. The unit is usually in a wristwatch-like package wo ...
for motor movements.


Brain waves in sleep

The electrical activity seen on an EEG represents brain waves. The amplitude of EEG waves at a particular frequency corresponds to various points in the sleep-wake cycle, such as being asleep, being awake, or falling asleep. Alpha, beta, theta, gamma, and delta waves are all seen in the different stages of sleep. Each waveform maintains a different frequency and amplitude. Alpha waves are seen when a person is in a resting state, but is still fully conscious. Their eyes may be closed and all of their body is resting and relatively still, where the body is starting to slow down. Beta waves take over alpha waves when a person is at attention, as they might be completing a task or concentrating on something. Beta waves consist of the highest of frequencies and the lowest of amplitude, and occur when a person is fully alert. Alpha and beta waves are the only waves seen when a person is in an awake state. Gamma waves are seen when a person is highly focused on a task or using all their concentration. Theta waves occur during the period of a person being awake, and they continue to transition into Stage 1 of sleep and in stage 2. Delta waves are seen in stages 3 and 4 of sleep when a person is in their deepest of sleep.


Non-REM and REM sleep

Sleep is divided into two broad types: non-rapid eye movement (non-REM or NREM) sleep and rapid eye movement (REM) sleep. Non-REM and REM sleep are so different that physiologists identify them as distinct behavioral states. Non-REM sleep occurs first and after a transitional period is called slow-wave sleep or deep sleep. During this phase, body temperature and heart rate fall, and the brain uses less energy. REM sleep, also known as paradoxical sleep, represents a smaller portion of total sleep time. It is the main occasion for dreams (or
nightmare A nightmare, also known as a bad dream, Retrieved 11 July 2016. is an unpleasant dream that can cause a strong emotional response from the mind, typically fear but also despair, anxiety or great sadness. The dream may contain situations of ...
s), and is associated with desynchronized and fast brain waves, eye movements, loss of muscle tone, and suspension of
homeostasis In biology, homeostasis ( British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism and ...
. The
sleep cycle The sleep cycle is an oscillation between the slow-wave and REM (paradoxical) phases of sleep. It is sometimes called the ultradian sleep cycle, sleep–dream cycle, or REM-NREM cycle, to distinguish it from the circadian alternation between sle ...
of alternate NREM and REM sleep takes an average of 90 minutes, occurring 4–6 times in a good night's sleep. The
American Academy of Sleep Medicine The American Academy of Sleep Medicine (AASM) is a United States professional society for the medical subspecialty of sleep medicine which includes disorders of circadian rhythms. It was established in 1975. The organization's functions include ...
(AASM) divides NREM into three stages: N1, N2, and N3, the last of which is also called delta sleep or slow-wave sleep. The whole period normally proceeds in the order: N1 → N2 → N3 → N2 → REM. REM sleep occurs as a person returns to stage 2 or 1 from a deep sleep. There is a greater amount of deep sleep (stage N3) earlier in the night, while the proportion of REM sleep increases in the two cycles just before natural awakening.


Awakening

Awakening can mean the end of sleep, or simply a moment to survey the environment and readjust body position before falling back asleep. Sleepers typically awaken soon after the end of a REM phase or sometimes in the middle of REM. Internal circadian indicators, along with a successful reduction of homeostatic sleep need, typically bring about awakening and the end of the sleep cycle. Awakening involves heightened electrical activation in the brain, beginning with the
thalamus The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all direct ...
and spreading throughout the cortex. On a typical night of sleep, there is not much time that is spent in the waking state. In various sleep studies that have been conducted using the electroencephalography, it has been found that females are awake for 0-1% during their nightly sleep while males are awake for 0-2% during that time. In adults, wakefulness increases, especially in later cycles. One study found 3% awake time in the first ninety-minute sleep cycle, 8% in the second, 10% in the third, 12% in the fourth, and 13–14% in the fifth. Most of this awake time occurred shortly after REM sleep. Today, many humans wake up with an
alarm clock An alarm clock (or sometimes just an alarm) is a clock that is designed to alert an individual or group of individuals at a specified time. The primary function of these clocks is to awaken people from their night's sleep or short naps; they ar ...
; however, people can also reliably wake themselves up at a specific time with no need for an alarm. Many sleep quite differently on workdays versus days off, a pattern which can lead to chronic circadian desynchronization. Many people regularly look at television and other screens before going to bed, a factor which may exacerbate disruption of the circadian cycle. Scientific studies on sleep have shown that sleep stage at awakening is an important factor in amplifying
sleep inertia Sleep inertia is a physiological state of impaired cognitive and sensory-motor performance that is present immediately after awakening. It persists during the transition of sleep to wakefulness, where an individual will experience feelings of drows ...
.


Timing

Sleep timing is controlled by the
circadian clock A circadian clock, or circadian oscillator, is a biochemical oscillator that cycles with a stable phase and is synchronized with solar time. Such a clock's ''in vivo'' period is necessarily almost exactly 24 hours (the earth's current solar day ...
(Process C), sleep-wake
homeostasis In biology, homeostasis ( British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism and ...
(Process S), and to some extent by the individual will.


Circadian clock

402px, The human " biological clock" Sleep timing depends greatly on
hormonal A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are required f ...
signals from the circadian clock, or Process C, a complex neurochemical system which uses signals from an organism's environment to recreate an internal day–night rhythm. Process C counteracts the homeostatic drive for sleep during the day (in diurnal animals) and augments it at night. The
suprachiasmatic nucleus The suprachiasmatic nucleus or nuclei (SCN) is a tiny region of the brain in the hypothalamus, situated directly above the optic chiasm. It is responsible for controlling circadian rhythms. The neuronal and hormonal activities it generates regu ...
(SCN), a brain area directly above the
optic chiasm In neuroanatomy, the optic chiasm, or optic chiasma (; , ), is the part of the brain where the optic nerves cross. It is located at the bottom of the brain immediately inferior to the hypothalamus. The optic chiasm is found in all vertebrat ...
, is presently considered the most important nexus for this process; however, secondary clock systems have been found throughout the body. An organism whose circadian clock exhibits a regular rhythm corresponding to outside signals is said to be '' entrained''; an entrained rhythm persists even if the outside signals suddenly disappear. If an entrained human is isolated in a bunker with constant light or darkness, he or she will continue to experience rhythmic increases and decreases of body temperature and melatonin, on a period that slightly exceeds 24 hours. Scientists refer to such conditions as free-running of the circadian rhythm. Under natural conditions, light signals regularly adjust this period downward, so that it corresponds better with the exact 24 hours of an Earth day. The circadian clock exerts constant influence on the body, affecting
sinusoidal A sine wave, sinusoidal wave, or just sinusoid is a mathematical curve defined in terms of the '' sine'' trigonometric function, of which it is the graph. It is a type of continuous wave and also a smooth periodic function. It occurs often i ...
oscillation of
body temperature Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperatur ...
between roughly 36.2 °C and 37.2 °C. The suprachiasmatic nucleus itself shows conspicuous oscillation activity, which intensifies during subjective day (i.e., the part of the rhythm corresponding with daytime, whether accurately or not) and drops to almost nothing during subjective night. The circadian pacemaker in the suprachiasmatic nucleus has a direct neural connection to the
pineal gland The pineal gland, conarium, or epiphysis cerebri, is a small endocrine gland in the brain of most vertebrates. The pineal gland produces melatonin, a serotonin-derived hormone which modulates sleep patterns in both circadian and seasonal cy ...
, which releases the hormone
melatonin Melatonin is a natural product found in plants and animals. It is primarily known in animals as a hormone released by the pineal gland in the brain at night, and has long been associated with control of the sleep–wake cycle. In vertebrat ...
at night.
Cortisol Cortisol is a steroid hormone, in the glucocorticoid class of hormones. When used as a medication, it is known as hydrocortisone. It is produced in many animals, mainly by the '' zona fasciculata'' of the adrenal cortex in the adrenal g ...
levels typically rise throughout the night, peak in the awakening hours, and diminish during the day. Circadian
prolactin Prolactin (PRL), also known as lactotropin, is a protein best known for its role in enabling mammals to produce milk. It is influential in over 300 separate processes in various vertebrates, including humans. Prolactin is secreted from the pi ...
secretion begins in the late afternoon, especially in women, and is subsequently augmented by sleep-induced secretion, to peak in the middle of the night. Circadian rhythm exerts some influence on the nighttime secretion of growth hormone. The circadian rhythm influences the ideal timing of a restorative sleep episode. Sleepiness increases during the night. REM sleep occurs more during body temperature minimum within the circadian cycle, whereas slow-wave sleep can occur more independently of circadian time. The internal circadian clock is profoundly influenced by changes in light, since these are its main clues about what time it is. Exposure to even small amounts of light during the night can suppress melatonin secretion, and increase body temperature and wakefulness. Short pulses of light, at the right moment in the circadian cycle, can significantly 'reset' the internal clock. Blue light, in particular, exerts the strongest effect, leading to concerns that electronic media use before bed may interfere with sleep. Modern humans often find themselves desynchronized from their internal circadian clock, due to the requirements of work (especially
night shifts Shift work is an employment practice designed to make use of, or provide service across, all 24 hours of the clock each day of the week (often abbreviated as ''24/7''). The practice typically sees the day divided into shifts, set periods of ...
), long-distance travel, and the influence of universal indoor lighting. Even if they have sleep debt, or feel sleepy, people can have difficulty staying asleep at the peak of their circadian cycle. Conversely, they can have difficulty waking up in the trough of the cycle. A healthy young adult entrained to the sun will (during most of the year) fall asleep a few hours after sunset, experience body temperature minimum at 6 a.m., and wake up a few hours after sunrise.


Process S

Generally speaking, the longer an organism is awake, the more it feels a need to sleep ("sleep debt"). This driver of sleep is referred to as Process S. The balance between sleeping and waking is regulated by a process called
homeostasis In biology, homeostasis ( British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism and ...
. Induced or perceived lack of sleep is called
sleep deprivation Sleep deprivation, also known as sleep insufficiency or sleeplessness, is the condition of not having adequate duration and/or quality of sleep to support decent alertness, performance, and health. It can be either chronic or acute and may vary ...
. Process S is driven by the depletion of
glycogen Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. The polysaccharide structure represents the main storage form of glucose in the body. Glycogen functions as one of ...
and accumulation of
adenosine Adenosine (symbol A) is an organic compound that occurs widely in nature in the form of diverse derivatives. The molecule consists of an adenine attached to a ribose via a β-N9- glycosidic bond. Adenosine is one of the four nucleoside building ...
in the forebrain that disinhibits the
ventrolateral preoptic nucleus The ventrolateral preoptic nucleus (VLPO), also known as the intermediate nucleus of the preoptic area (IPA), is a small cluster of neurons situated in the anterior hypothalamus, sitting just above and to the side of the optic chiasm in the brain ...
, allowing for inhibition of the
ascending reticular activating system The reticular formation is a set of interconnected nuclei that are located throughout the brainstem. It is not anatomically well defined, because it includes neurons located in different parts of the brain. The neurons of the reticular formati ...
. Sleep deprivation tends to cause slower brain waves in the
frontal cortex The frontal lobe is the largest of the four major lobes of the brain in mammals, and is located at the front of each cerebral hemisphere (in front of the parietal lobe and the temporal lobe). It is parted from the parietal lobe by a groove be ...
, shortened attention span, higher anxiety, impaired memory, and a grouchy mood. Conversely, a well-rested organism tends to have improved memory and mood. Neurophysiological and functional
imaging studies Medical imaging is the technique and process of imaging the interior of a body for clinical analysis and medical intervention, as well as visual representation of the function of some organs or tissues (physiology). Medical imaging seeks to rev ...
have demonstrated that frontal regions of the brain are particularly responsive to homeostatic sleep pressure. There is disagreement on how much sleep debt is possible to accumulate, and whether sleep debt is accumulated against an individual's average sleep or some other benchmark. It is also unclear whether the prevalence of sleep debt among adults has changed appreciably in the
industrialized world A developed country (or industrialized country, high-income country, more economically developed country (MEDC), advanced country) is a sovereign state that has a high quality of life, developed economy and advanced technological infrastru ...
in recent decades. Sleep debt does show some evidence of being cumulative. Subjectively, however, humans seem to reach maximum sleepiness 30 hours after waking. It is likely that in
Western societies The Western world, also known as the West, primarily refers to the various nations and states in the regions of Europe, North America, and Oceania.
, children are sleeping less than they previously have. One neurochemical indicator of sleep debt is
adenosine Adenosine (symbol A) is an organic compound that occurs widely in nature in the form of diverse derivatives. The molecule consists of an adenine attached to a ribose via a β-N9- glycosidic bond. Adenosine is one of the four nucleoside building ...
, a neurotransmitter that inhibits many of the bodily processes associated with wakefulness. Adenosine levels increase in the cortex and basal forebrain during prolonged wakefulness, and decrease during the sleep-recovery period, potentially acting as a homeostatic regulator of sleep.
Coffee Coffee is a drink prepared from roasted coffee beans. Darkly colored, bitter, and slightly acidic, coffee has a stimulating effect on humans, primarily due to its caffeine content. It is the most popular hot drink in the world. Seeds of ...
, tea, and other sources of
caffeine Caffeine is a central nervous system (CNS) stimulant of the methylxanthine chemical classification, class. It is mainly recreational drug use, used recreationally as a Nootropic, cognitive enhancer, increasing alertness and attentional perfor ...
temporarily block the effect of adenosine, prolong sleep latency, and reduce total sleep time and quality.


Social timing

Humans are also influenced by aspects of ''social time'', such as the hours when other people are awake, the hours when work is required, the time on clocks, etc.
Time zone A time zone is an area which observes a uniform standard time for legal, commercial and social purposes. Time zones tend to follow the boundaries between countries and their subdivisions instead of strictly following longitude, because it ...
s, standard times used to unify the timing for people in the same area, correspond only approximately to the natural rising and setting of the sun. An extreme example of the approximate nature of time zones is China, a country which used to span five time zones and now officially uses only one (UTC+8).


Distribution

In
polyphasic sleep Biphasic sleep (or ''diphasic'', ''bifurcated'', or bimodal sleep) is the practice of sleeping during two periods over the course of 24 hours, while polyphasic sleep refers to sleeping multiple times—usually more than two. Each of these is in ...
, an organism sleeps several times in a 24-hour cycle, whereas in monophasic sleep this occurs all at once. Under experimental conditions, humans tend to alternate more frequently between sleep and wakefulness (i.e., exhibit more polyphasic sleep) if they have nothing better to do. Given a 14-hour period of darkness in experimental conditions, humans tended towards bimodal sleep, with two sleep periods concentrated at the beginning and at the end of the dark time. Bimodal sleep in humans was more common before the
industrial revolution The Industrial Revolution was the transition to new manufacturing processes in Great Britain, continental Europe, and the United States, that occurred during the period from around 1760 to about 1820–1840. This transition included going f ...
. Different characteristic sleep patterns, such as the familiarly so-called " early bird" and " night owl", are called ''
chronotype A chronotype is the behavioral manifestation of underlying circadian rhythm's myriad of physical processes. A person's chronotype is the propensity for the individual to sleep at a particular time during a 24-hour period. ''Eveningness'' (delayed ...
s''. Genetics and sex have some influence on chronotype, but so do habits. Chronotype is also liable to change over the course of a person's lifetime. Seven-year-olds are better disposed to wake up early in the morning than are fifteen-year-olds. Chronotypes far outside the normal range are called circadian rhythm sleep disorders.


Naps

Naps are short periods of sleep that one might take during the daytime, often in order to get the necessary amount of rest. Napping is often associated with childhood, but around one-third of American adults partake in it daily. The optimal nap duration is around 10–20 minutes, as researchers have proven that it takes at least 30 minutes to enter slow-wave sleep, the deepest period of sleep.Fry, A. (9 October 2020). ''Napping: Health Benefits & Tips for your best nap''. Sleep Foundation. Retrieved 14 November 2021, from https://www.sleepfoundation.org/sleep-hygiene/napping. Napping too long and entering the slow wave cycles can make it difficult to awake from the nap and leave one feeling unrested. This period of drowsiness is called
sleep inertia Sleep inertia is a physiological state of impaired cognitive and sensory-motor performance that is present immediately after awakening. It persists during the transition of sleep to wakefulness, where an individual will experience feelings of drows ...
. The
siesta A ''siesta'' (from Spanish, pronounced and meaning "nap") is a short nap taken in the early afternoon, often after the midday meal. Such a period of sleep is a common tradition in some countries, particularly those in warm-weather zones. The ...
habit has recently been associated with a 37% lower coronary mortality, possibly due to reduced cardiovascular stress mediated by daytime sleep. Short naps at mid-day and mild evening exercise were found to be effective for improved sleep, cognitive tasks, and mental health in elderly people.


Genetics

Monozygotic (identical) but not dizygotic (fraternal) twins tend to have similar sleep habits. Neurotransmitters, molecules whose production can be traced to specific genes, are one genetic influence on sleep that can be analyzed. The circadian clock has its own set of genes. Genes which may influence sleep include
ABCC9 ATP-binding cassette, sub-family C member 9 (ABCC9) also known as sulfonylurea receptor 2 (SUR2) is an ATP-binding cassette transporter that in humans is encoded by the ''ABCC9'' gene. Function The protein encoded by this gene is a member of ...
,
DEC2 "Basic helix-loop-helix family, member e41", or BHLHE41, is a gene that encodes a basic helix-loop-helix transcription factor repressor protein in various tissues of both humans and mice. It is also known as DEC2, hDEC2, and SHARP1, and was previ ...
,
Dopamine receptor D2 Dopamine receptor D2, also known as D2R, is a protein that, in humans, is encoded by the ''DRD2'' gene. After work from Paul Greengard's lab had suggested that dopamine receptors were the site of action of antipsychotic drugs, several groups, i ...
and variants near PAX 8 and
VRK2 Serine/threonine-protein kinase VRK2 is an enzyme that in humans is encoded by the ''VRK2'' gene. This gene encodes a member of the vaccinia-related kinase (VRK) family of serine/threonine protein kinases. This gene is widely expressed in human ...
. While the latter have been found in a GWAS study that primarily detects correlations (but not necessarily causation), other genes have been shown to have a more direct effect. For instance, mice lacking
dihydropyrimidine dehydrogenase In enzymology, a dihydropyrimidine dehydrogenase (NADP+) () is an enzyme that catalyzes the chemical reaction :5,6-dihydrouracil + NADP+ \rightleftharpoons uracil + NADPH + H+ Thus, the two substrates of this enzyme are 5,6-dihydrouracil an ...
(Dpyd) had 78.4 min less sleep during the lights-off period than wild-type mice. Dpyd encodes the rate-limiting
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
in the metabolic pathway that catabolizes
uracil Uracil () (symbol U or Ura) is one of the four nucleobases in the nucleic acid RNA. The others are adenine (A), cytosine (C), and guanine (G). In RNA, uracil binds to adenine via two hydrogen bonds. In DNA, the uracil nucleobase is replaced b ...
and
thymidine Thymidine (symbol dT or dThd), also known as deoxythymidine, deoxyribosylthymine, or thymine deoxyriboside, is a pyrimidine deoxynucleoside. Deoxythymidine is the DNA nucleoside T, which pairs with deoxyadenosine (A) in double-stranded DNA. ...
to β-
alanine Alanine (symbol Ala or A), or α-alanine, is an α-amino acid that is used in the biosynthesis of proteins. It contains an amine group and a carboxylic acid group, both attached to the central carbon atom which also carries a methyl group side ...
, an inhibitory
neurotransmitter A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell. Neu ...
. This also supports the role of β-alanine as a neurotransmitter that promotes sleep in mice.


Quality

The quality of sleep may be evaluated from an objective and a subjective point of view. Objective sleep quality refers to how difficult it is for a person to fall asleep and remain in a sleeping state, and how many times they wake up during a single night. Poor sleep quality disrupts the cycle of transition between the different stages of sleep. Subjective sleep quality in turn refers to a sense of being rested and regenerated after awaking from sleep. A study by A. Harvey et al. (2002) found that insomniacs were more demanding in their evaluations of sleep quality than individuals who had no sleep problems. Homeostatic sleep propensity (the need for sleep as a function of the amount of time elapsed since the last adequate sleep episode) must be balanced against the circadian element for satisfactory sleep. Along with corresponding messages from the circadian clock, this tells the body it needs to sleep. The timing is correct when the following two circadian markers occur after the middle of the sleep episode and before awakening: maximum concentration of the hormone melatonin, and minimum core body temperature.


Ideal duration

Human sleep-needs vary by age and amongst individuals; sleep is considered to be adequate when there is no daytime sleepiness or dysfunction. Moreover, self-reported sleep duration is only moderately correlated with actual sleep time as measured by
actigraphy Actigraphy is a non-invasive method of monitoring human rest/activity cycles. A small actigraph unit, also called an actimetry sensor, is worn for a week or more to measure gross motor activity. The unit is usually in a wristwatch-like package wo ...
, and those affected with
sleep state misperception Sleep state misperception (SSM) is a term in the International Classification of Sleep Disorders (ICSD) most commonly used for people who mistakenly perceive their sleep as wakefulness,Minecan, Daniela, and Antonio Culebras. http://www.medlink.com ...
may typically report having slept only four hours despite having slept a full eight hours.Insomnia Causes
. Healthcommunities.com. Original Publication: 1 December 2000, Updated: 1 December 2007.
Researchers have found that sleeping 6–7 hours each night correlates with longevity and cardiac health in humans, though many underlying factors may be involved in the causality behind this relationship. Sleep difficulties are furthermore associated with psychiatric disorders such as depression,
alcoholism Alcoholism is, broadly, any drinking of alcohol that results in significant mental or physical health problems. Because there is disagreement on the definition of the word ''alcoholism'', it is not a recognized diagnostic entity. Predomi ...
, and
bipolar disorder Bipolar disorder, previously known as manic depression, is a mental disorder characterized by periods of Depression (mood), depression and periods of abnormally elevated Mood (psychology), mood that last from days to weeks each. If the elevat ...
. Up to 90 percent of adults with depression are found to have sleep difficulties. Dysregulation detected by EEG includes disturbances in sleep continuity, decreased delta sleep and altered REM patterns with regard to latency, distribution across the night and density of eye movements. Sleep duration can also vary according to season. Up to 90% of people report longer sleep duration in winter, which may lead to more pronounced
seasonal affective disorder Seasonal affective disorder (SAD) is a mood disorder subset, in which people who have normal mental health throughout most of the year exhibit depressive symptoms at the same time each year. Common symptoms include sleeping too much, having li ...
.


Children

By the time infants reach the age of two, their brain size has reached 90 percent of an adult-sized brain; a majority of this brain growth has occurred during the period of life with the highest rate of sleep. The hours that children spend asleep influence their ability to perform on cognitive tasks. Children who sleep through the night and have few night waking episodes have higher cognitive attainments and easier temperaments than other children. Sleep also influences language development. To test this, researchers taught infants a faux language and observed their recollection of the rules for that language. Infants who slept within four hours of learning the language could remember the language rules better, while infants who stayed awake longer did not recall those rules as well. There is also a relationship between infants' vocabulary and sleeping: infants who sleep longer at night at 12 months have better vocabularies at 26 months. Children can greatly benefit from a structured bedtime routine. This can look differently among families, but will generally consist of a set of rituals such as reading a bedtime story, a bath, brushing teeth, and can also include a show of affection from the parent to the child such a hug or kiss before bed. A bedtime routine will also include a consistent time that the child is expected to be in bed ready for sleep. Having a reliable bedtime routine can help improve a child's quality of sleep as well as prepare them to make and keep healthy
sleep hygiene Sleep hygiene is a behavioral and environmental practice developed in the late 1970s as a method to help people with mild to moderate insomnia. Clinicians assess the sleep hygiene of people with insomnia and other conditions, such as depression, ...
habits in the future.


Recommended duration

Children need many hours of sleep per day in order to develop and function properly: up to 18 hours for
newborn An infant or baby is the very young offspring of human beings. ''Infant'' (from the Latin word ''infans'', meaning 'unable to speak' or 'speechless') is a formal or specialised synonym for the common term ''baby''. The terms may also be used to ...
babies, with a declining rate as a child ages. Early in 2015, after a two-year study, the
National Sleep Foundation The National Sleep Foundation (NSF) is an American non-profit, charitable organization. Founded in 1990, it's stated goal is to provide expert information on health-related issues concerning sleep. Research NSF Sleep Duration Recommendations ...
in the US announced newly revised recommendations as shown in the table below.


Functions


Restoration

The human organism physically restores itself during sleep, occurring mostly during slow-wave sleep during which body temperature, heart rate, and brain oxygen consumption decrease. In both the brain and body, the reduced rate of
metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run ...
enables countervailing restorative processes.Raymond Cespuglio, Damien Colas, & Sabine Gautier-Sauvigné, "Energy Processes Underlying the Sleep Wake Cycle"; Chapter 1 in Parmeggiani & Velluti (2005). The brain requires sleep for restoration, whereas these processes can take place during quiescent waking in the rest of the body. The essential function of sleep may be its restorative effect on the brain: "Sleep is of the brain, by the brain and for the brain." This theory is strengthened by the fact that sleep is observed to be a necessary behavior across most of the animal kingdom, including some of the least evolved animals which have no need for other functions of sleep, such as memory consolidation or dreaming. While awake, brain metabolism generates end products, such as
reactive oxygen species In chemistry, reactive oxygen species (ROS) are highly reactive chemicals formed from diatomic oxygen (). Examples of ROS include peroxides, superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen. The reduction of molecular oxygen () p ...
, which may be damaging to brain cells and inhibit their proper function. During sleep, metabolic rates decrease and reactive oxygen species generation is reduced, enabling restorative processes. The sleeping brain has been shown to remove metabolic end products at a faster rate than during an awake state. The mechanism for this removal appears to be the
glymphatic system The glymphatic system (or glymphatic clearance pathway, or paravascular system) was described and named in 2013 as a system for waste clearance in the central nervous system (CNS) of vertebrates. According to this model, cerebrospinal fluid (CSF) ...
. Sleep may facilitate the synthesis of molecules that help repair and protect the brain from metabolic end products generated during waking.
Anabolic Anabolism () is the set of metabolic pathways that construct molecules from smaller units. These reactions require energy, known also as an endergonic process. Anabolism is the building-up aspect of metabolism, whereas catabolism is the breakin ...
hormones, such as
growth hormone Growth hormone (GH) or somatotropin, also known as human growth hormone (hGH or HGH) in its human form, is a peptide hormone that stimulates growth, cell reproduction, and cell regeneration in humans and other animals. It is thus important in ...
s, are secreted preferentially during sleep. The brain concentration of
glycogen Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. The polysaccharide structure represents the main storage form of glucose in the body. Glycogen functions as one of ...
increases during sleep, and is depleted through metabolism during wakefulness. The effect of sleep duration on somatic growth is not completely known. One study recorded growth, height, and weight, as correlated to parent-reported time in bed in 305 children over a period of nine years (age 1–10). It was found that "the variation of sleep duration among children does not seem to have an effect on growth." Slow-wave sleep affects
growth hormone Growth hormone (GH) or somatotropin, also known as human growth hormone (hGH or HGH) in its human form, is a peptide hormone that stimulates growth, cell reproduction, and cell regeneration in humans and other animals. It is thus important in ...
levels in adult men. During eight hours of sleep, men with a high percentage of slow-wave sleep (SWS) (average 24%) also had high growth hormone secretion, while subjects with a low percentage of SWS (average 9%) had low growth hormone secretion.


Memory processing

It has been widely accepted that sleep must support the formation of long-term memory, and generally increasing previous learning and experiences recalls. However, its benefit seems to depend on the phase of sleep and the type of memory. For example, declarative and procedural memory-recall tasks applied over early and late nocturnal sleep, as well as wakefulness controlled conditions, have been shown that declarative memory improves more during early sleep (dominated by SWS) while procedural memory during late sleep (dominated by REM sleep) does so. With regard to declarative memory, the functional role of SWS has been associated with hippocampal replays of previously encoded neural patterns that seem to facilitate long-term memory consolidation. This assumption is based on the active system consolidation hypothesis, which states that repeated reactivations of newly encoded information in the hippocampus during slow oscillations in NREM sleep mediate the stabilization and gradual integration of declarative memory with pre-existing knowledge networks on the cortical level. It assumes the hippocampus might hold information only temporarily and in a fast-learning rate, whereas the neocortex is related to long-term storage and a slow-learning rate. This dialogue between the hippocampus and neocortex occurs in parallel with hippocampal sharp-wave ripples and thalamo-cortical spindles, synchrony that drives the formation of the spindle-ripple event which seems to be a prerequisite for the formation of long-term memories. Reactivation of memory also occurs during wakefulness and its function is associated with serving to update the reactivated memory with newly encoded information, whereas reactivations during SWS are presented as crucial for memory stabilization. Based on targeted memory reactivation (TMR) experiments that use associated memory cues to triggering memory traces during sleep, several studies have been reassuring the importance of nocturnal reactivations for the formation of persistent memories in neocortical networks, as well as highlighting the possibility of increasing people's memory performance at declarative recalls. Furthermore, nocturnal reactivation seems to share the same neural oscillatory patterns as reactivation during wakefulness, processes which might be coordinated by theta activity. During wakefulness, theta oscillations have been often related to successful performance in memory tasks, and cued memory reactivations during sleep have been showing that theta activity is significantly stronger in subsequent recognition of cued stimuli as compared to uncued ones, possibly indicating a strengthening of memory traces and lexical integration by cuing during sleep. However, the beneficial effect of TMR for memory consolidation seems to occur only if the cued memories can be related to prior knowledge.


Dreaming

During sleep, especially REM sleep, humans tend to experience dreams. These are elusive and mostly unpredictable first-person experiences which seem logical and realistic to the dreamer while they are in progress, despite their frequently bizarre, irrational, and/or surreal qualities that become apparent when assessed after waking. Dreams often seamlessly incorporate concepts, situations, people, and objects within a person's mind that would not normally go together. They can include apparent sensations of all types, especially vision and movement.J. Alan Hobson, Edward F. Pace-Scott, & Robert Stickgold (2000), "Dreaming and the brain: Toward a cognitive neuroscience of conscious states", ''Behavioral and Brain Sciences'' 23. Dreams tend to rapidly fade from memory after waking. Some people choose to keep a
dream journal A dream diary (or dream journal) is a diary in which dream experiences are recorded. A dream diary might include a record of nightly dreams, personal reflections and waking dream experiences. It is often used in the study of dreams and psycholo ...
, which they believe helps them build dream recall and facilitate the ability to experience
lucid dreams A lucid dream is a type of dream in which the dreamer becomes aware that they are dreaming while dreaming. During a lucid dream, the dreamer may gain some amount of control over the dream characters, narrative, or environment; however, this is n ...
. People have proposed many
hypotheses A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous obse ...
about the functions of dreaming.
Sigmund Freud Sigmund Freud ( , ; born Sigismund Schlomo Freud; 6 May 1856 – 23 September 1939) was an Austrian neurologist and the founder of psychoanalysis, a clinical method for evaluating and treating pathologies explained as originating in conflicts i ...
postulated that dreams are the symbolic expression of frustrated desires that have been relegated to the
unconscious mind The unconscious mind (or the unconscious) consists of the processes in the mind which occur automatically and are not available to introspection and include thought processes, memories, interests, and motivations. Even though these processes exi ...
, and he used
dream interpretation Dream interpretation is the process of assigning meaning to dreams. Although associated with some forms of psychotherapy, there is no reliable evidence that understanding or interpreting dreams has a positive impact on one's mental health. In m ...
in the form of
psychoanalysis PsychoanalysisFrom Greek: + . is a set of theories and therapeutic techniques"What is psychoanalysis? Of course, one is supposed to answer that it is many things — a theory, a research method, a therapy, a body of knowledge. In what might ...
in attempting to uncover these desires. Counterintuitively, penile erections during sleep are not more frequent during sexual dreams than during other dreams. The
parasympathetic nervous system The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part o ...
experiences increased activity during REM sleep which may cause erection of the penis or clitoris. In males, 80% to 95% of REM sleep is normally accompanied by partial to full penile erection, while only about 12% of men's dreams contain sexual content. John Allan Hobson and
Robert McCarley Robert W. McCarley, MD, (1937–2017) was Chair and Professor of Psychiatry at Harvard Medical School and the VA Boston Healthcare System. He is also Director of the Laboratory of Neuroscience located at the Brockton VA Medical Center and the ...
propose that dreams are caused by the random firing of neurons in the
cerebral cortex The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consisting o ...
during the REM period. Neatly, this theory helps explain the irrationality of the mind during REM periods, as, according to this theory, the
forebrain In the anatomy of the brain of vertebrates, the forebrain or prosencephalon is the rostral (forward-most) portion of the brain. The forebrain (prosencephalon), the midbrain (mesencephalon), and hindbrain (rhombencephalon) are the three primary ...
then creates a story in an attempt to reconcile and make sense of the nonsensical sensory information presented to it. This would explain the odd nature of many dreams.


Disorders


Insomnia

Insomnia is a general term for difficulty falling asleep and/or staying asleep. Insomnia is the most common sleep problem, with many adults reporting occasional insomnia, and 10–15% reporting a chronic condition. Insomnia can have many different causes, including
psychological stress In psychology, stress is a feeling of emotional strain and pressure. Stress is a type of psychological pain. Small amounts of stress may be beneficial, as it can improve athletic performance, motivation and reaction to the environment. Excess ...
, a poor sleep environment, an inconsistent sleep schedule, or excessive mental or physical stimulation in the hours before bedtime. Insomnia is often treated through behavioral changes like keeping a regular sleep schedule, avoiding stimulating or stressful activities before bedtime, and cutting down on stimulants such as caffeine. The sleep environment may be improved by installing heavy drapes to shut out all sunlight, and keeping computers, televisions, and work materials out of the sleeping area. A 2010 review of published scientific research suggested that exercise generally improves sleep for most people, and helps sleep disorders such as insomnia. The optimum time to exercise ''may'' be 4 to 8 hours before bedtime, though exercise at any time of day is beneficial, with the exception of heavy exercise taken shortly before bedtime, which may disturb sleep. However, there is insufficient evidence to draw detailed conclusions about the relationship between exercise and sleep.
Nonbenzodiazepine Nonbenzodiazepines (), sometimes referred to colloquially as Z-drugs (as many of their names begin with the letter "z"), are a class of psychoactive drugs that are very benzodiazepine-like in nature. They are used in the treatment of sleep proble ...
sleeping medications such as Ambien, Imovane, and
Lunesta Eszopiclone, sold under the brand-name Lunesta among others such as Night Calm in Egypt, is a medication used in the treatment of insomnia. Evidence supports slight to moderate benefit up to six months. It is taken orally. Common side effects i ...
(also known as "Z-drugs"), while initially believed to be better and safer than earlier generations of including
benzodiazepine Benzodiazepines (BZD, BDZ, BZs), sometimes called "benzos", are a class of depressant drugs whose core chemical structure is the fusion of a benzene ring and a diazepine ring. They are prescribed to treat conditions such as anxiety disorders, ...
are now known to be almost entirely the same as benzodiazepines in terms of their
pharmacodynamics Pharmacodynamics (PD) is the study of the biochemical and physiologic effects of drugs (especially pharmaceutical drugs). The effects can include those manifested within animals (including humans), microorganisms, or combinations of organisms ...
, differing only at the molecular level in their chemical structure, and therefore exhibit similar benefits, side-effects, and risks.
White noise In signal processing, white noise is a random signal having equal intensity at different frequencies, giving it a constant power spectral density. The term is used, with this or similar meanings, in many scientific and technical disciplines ...
appears to be a promising treatment for
insomnia Insomnia, also known as sleeplessness, is a sleep disorder in which people have trouble sleeping. They may have difficulty falling asleep, or staying asleep as long as desired. Insomnia is typically followed by daytime sleepiness, low energy, ...
.


Sleep health

Low quality sleep has been linked with health conditions like
cardiovascular disease Cardiovascular disease (CVD) is a class of diseases that involve the heart or blood vessels. CVD includes coronary artery diseases (CAD) such as angina and myocardial infarction (commonly known as a heart attack). Other CVDs include stroke, hea ...
,
obesity Obesity is a medical condition, sometimes considered a disease, in which excess body fat has accumulated to such an extent that it may negatively affect health. People are classified as obese when their body mass index (BMI)—a person's ...
, and
mental illness A mental disorder, also referred to as a mental illness or psychiatric disorder, is a behavioral or mental pattern that causes significant distress or impairment of personal functioning. Such features may be persistent, relapsing and remitt ...
. While poor sleep is common among those with cardiovascular disease, some research indicates that poor sleep can be a contributing cause. Short sleep duration of less than seven hours is correlated with
coronary heart disease Coronary artery disease (CAD), also called coronary heart disease (CHD), ischemic heart disease (IHD), myocardial ischemia, or simply heart disease, involves the reduction of blood flow to the heart muscle due to build-up of atherosclerotic pl ...
and increased risk of death from coronary heart disease. Sleep duration greater than nine hours is also correlated with coronary heart disease, as well as
stroke A stroke is a disease, medical condition in which poor cerebral circulation, blood flow to the brain causes cell death. There are two main types of stroke: brain ischemia, ischemic, due to lack of blood flow, and intracranial hemorrhage, hemorr ...
and cardiovascular events. In both children and adults, short sleep duration is associated with an increased risk of obesity, with various studies reporting an increased risk of 45–55%. Other aspects of sleep health have been associated with obesity, including daytime napping, sleep timing, the variability of sleep timing, and low sleep efficiency. However, sleep duration is the most-studied for its impact on obesity. Sleep problems have been frequently viewed as a symptom of mental illness rather than a causative factor. However, a growing body of evidence suggests that they are both a cause and a symptom of mental illness. Insomnia is a significant predictor of
major depressive disorder Major depressive disorder (MDD), also known as clinical depression, is a mental disorder characterized by at least two weeks of pervasive low mood, low self-esteem, and loss of interest or pleasure in normally enjoyable activities. Intro ...
; a
meta-analysis A meta-analysis is a statistical analysis that combines the results of multiple scientific studies. Meta-analyses can be performed when there are multiple scientific studies addressing the same question, with each individual study reporting m ...
of 170,000 people showed that insomnia at the beginning of a study period indicated a more than the twofold increased risk for major depressive disorder. Some studies have also indicated correlation between insomnia and
anxiety Anxiety is an emotion which is characterized by an unpleasant state of inner turmoil and includes feelings of dread over anticipated events. Anxiety is different than fear in that the former is defined as the anticipation of a future threat wh ...
,
post-traumatic stress disorder Post-traumatic stress disorder (PTSD) is a mental and behavioral disorder that can develop because of exposure to a traumatic event, such as sexual assault, warfare, traffic collisions, child abuse, domestic violence, or other threats o ...
, and
suicide Suicide is the act of intentionally causing one's own death. Mental disorders (including depression, bipolar disorder, schizophrenia, personality disorders, anxiety disorders), physical disorders (such as chronic fatigue syndrome), and ...
. Sleep disorders can increase the risk of
psychosis Psychosis is a condition of the mind that results in difficulties determining what is real and what is not real. Symptoms may include delusions and hallucinations, among other features. Additional symptoms are incoherent speech and behavi ...
and worsen the severity of psychotic episodes. Sleep research also displays differences in race and class. Short sleep and poor sleep are observed more frequently in ethnic minorities than in whites. African-Americans report experiencing short durations of sleep five times more often than whites, possibly as a result of social and environmental factors. Black children and children in disadvantaged neighborhoods have much higher rates of sleep apnea than white children and respond more poorly to treatment. Sleep health can be improved through implementing good sleep hygiene habits. Having good sleep hygiene can help to improve your physical and mental health by providing your body with the necessary rejuvenation only restful sleep can provide. Some ways to improve sleep health include going to sleep at consistent times every night, avoiding any electronic devices such as televisions in the bedroom, getting adequate exercise throughout your day, and avoiding caffeine in the hours before going to sleep. Another way to greatly improve sleep hygiene is by creating a peaceful and relaxing sleep environment. Sleeping in a dark and clean room with things like a white noise maker can help facilitate restful sleep.


Drugs and diet

Drugs which induce sleep, known as
hypnotic Hypnotic (from Greek ''Hypnos'', sleep), or soporific drugs, commonly known as sleeping pills, are a class of (and umbrella term for) psychoactive drugs whose primary function is to induce sleep (or surgical anesthesiaWhen used in anesthesia ...
s, include
benzodiazepine Benzodiazepines (BZD, BDZ, BZs), sometimes called "benzos", are a class of depressant drugs whose core chemical structure is the fusion of a benzene ring and a diazepine ring. They are prescribed to treat conditions such as anxiety disorders, ...
s, although these interfere with REM;
Nonbenzodiazepine Nonbenzodiazepines (), sometimes referred to colloquially as Z-drugs (as many of their names begin with the letter "z"), are a class of psychoactive drugs that are very benzodiazepine-like in nature. They are used in the treatment of sleep proble ...
hypnotics such as
eszopiclone Eszopiclone, sold under the brand-name Lunesta among others such as Night Calm in Egypt, is a medication used in the treatment of insomnia. Evidence supports slight to moderate benefit up to six months. It is taken orally. Common side effects i ...
(Lunesta),
zaleplon Zaleplon, sold under the brand names Sonata among others, is a sedative- hypnotic, used to treat insomnia. It is a nonbenzodiazepine hypnotic from the pyrazolopyrimidine class. It is manufactured by King Pharmaceuticals and Gedeon Richte ...
(Sonata), and
zolpidem Zolpidem, sold under the brand name Ambien, among others, is a medication primarily used for the short-term treatment of sleeping problems. Guidelines recommend that it be used only after cognitive behavioral therapy for insomnia and behavior ...
(Ambien);
antihistamine Antihistamines are drugs which treat allergic rhinitis, common cold, influenza, and other allergies. Typically, people take antihistamines as an inexpensive, generic (not patented) drug that can be bought without a prescription and provide ...
s, such as
diphenhydramine Diphenhydramine (DPH) is an antihistamine and sedative mainly used to treat allergies, insomnia, and symptoms of the common cold. It is also less commonly used for tremor in parkinsonism, and nausea. It is taken by mouth, injected into ...
(Benadryl) and doxylamine; alcohol (ethanol), despite its
rebound effect The rebound effect, or rebound phenomenon, is the emergence or re-emergence of symptoms that were either absent or controlled while taking a medication, but appear when that same medication is discontinued, or reduced in dosage. In the case of re ...
later in the night and interference with REM;
barbiturate Barbiturates are a class of depressant drugs that are chemically derived from barbituric acid. They are effective when used medically as anxiolytics, hypnotics, and anticonvulsants, but have physical and psychological addiction potential a ...
s, which have the same problem;
melatonin Melatonin is a natural product found in plants and animals. It is primarily known in animals as a hormone released by the pineal gland in the brain at night, and has long been associated with control of the sleep–wake cycle. In vertebrat ...
, a component of the circadian clock, and released naturally at night by the
pineal gland The pineal gland, conarium, or epiphysis cerebri, is a small endocrine gland in the brain of most vertebrates. The pineal gland produces melatonin, a serotonin-derived hormone which modulates sleep patterns in both circadian and seasonal cy ...
; and
cannabis ''Cannabis'' () is a genus of flowering plants in the family Cannabaceae. The number of species within the genus is disputed. Three species may be recognized: '' Cannabis sativa'', '' C. indica'', and '' C. ruderalis''. Alternative ...
, which may also interfere with REM. Some
opioid Opioids are substances that act on opioid receptors to produce morphine-like effects. Medically they are primarily used for pain relief, including anesthesia. Other medical uses include suppression of diarrhea, replacement therapy for opioid use ...
s (including
morphine Morphine is a strong opiate that is found naturally in opium, a dark brown resin in poppies ('' Papaver somniferum''). It is mainly used as a pain medication, and is also commonly used recreationally, or to make other illicit opioids. T ...
,
codeine Codeine is an opiate and prodrug of morphine mainly used to treat pain, coughing, and diarrhea. It is also commonly used as a recreational drug. It is found naturally in the sap of the opium poppy, ''Papaver somniferum''. It is typically ...
,
heroin Heroin, also known as diacetylmorphine and diamorphine among other names, is a potent opioid mainly used as a recreational drug for its euphoric effects. Medical grade diamorphine is used as a pure hydrochloride salt. Various white and bro ...
, and
oxycodone Oxycodone, sold under various brand names such as Roxicodone and OxyContin (which is the extended release form), is a strong, semi-synthetic opioid used medically for treatment of moderate to severe pain. It is highly addictive and a commonly ...
) also induce sleep, and can disrupt sleep architecture and sleep stage distribution.
Stimulant Stimulants (also often referred to as psychostimulants or colloquially as uppers) is an overarching term that covers many drugs including those that increase activity of the central nervous system and the body, drugs that are pleasurable and inv ...
s, which inhibit sleep, include
caffeine Caffeine is a central nervous system (CNS) stimulant of the methylxanthine chemical classification, class. It is mainly recreational drug use, used recreationally as a Nootropic, cognitive enhancer, increasing alertness and attentional perfor ...
, an
adenosine Adenosine (symbol A) is an organic compound that occurs widely in nature in the form of diverse derivatives. The molecule consists of an adenine attached to a ribose via a β-N9- glycosidic bond. Adenosine is one of the four nucleoside building ...
antagonist;
amphetamine Amphetamine (contracted from alpha- methylphenethylamine) is a strong central nervous system (CNS) stimulant that is used in the treatment of attention deficit hyperactivity disorder (ADHD), narcolepsy, and obesity. It is also commonly used ...
,
methamphetamine Methamphetamine (contracted from ) is a potent central nervous system (CNS) stimulant that is mainly used as a recreational drug and less commonly as a second-line treatment for attention deficit hyperactivity disorder and obesity. Methamp ...
,
MDMA 3,4-Methylenedioxymethamphetamine (MDMA), commonly seen in tablet form (ecstasy) and crystal form (molly or mandy), is a potent empathogen–entactogen with stimulant properties primarily used for recreational purposes. The desire ...
, empathogen-entactogens, and related drugs;
cocaine Cocaine (from , from , ultimately from Quechua: ''kúka'') is a central nervous system (CNS) stimulant mainly used recreationally for its euphoric effects. It is primarily obtained from the leaves of two Coca species native to South Am ...
, which can alter the circadian rhythm, and
methylphenidate Methylphenidate, sold under the brand names Ritalin and Concerta among others, is the most widely prescribed central nervous system (CNS) stimulant medication used to treat attention deficit hyperactivity disorder (ADHD) and, to a lesser extent ...
, which acts similarly; and other analeptic drugs like
modafinil Modafinil, sold under the brand name Provigil among others, is a central nervous system (CNS) stimulant medication used to treat sleepiness due to narcolepsy, shift work sleep disorder, and obstructive sleep apnea. While it has seen off-la ...
and armodafinil with poorly understood mechanisms. Consuming high amounts of the stimulant caffeine can result in interrupted sleep patterns and sometimes sleep deprivation. This vicious cycle can result in drowsiness which can then result in a higher consumption of caffeine in order to stay awake the next day. This cycle can lead to decreased cognitive function and an overall feeling of fatigue. Some drugs may alter sleep architecture without inhibiting or inducing sleep. Drugs that amplify or inhibit endocrine and immune system secretions associated with certain sleep stages have been shown to alter sleep architecture. The growth hormone releasing hormone receptor agonist MK-677 has been shown to increase REM in older adults as well as stage IV sleep in younger adults by approximately 50%. Dietary and nutritional choices may affect sleep duration and quality. One 2016 review indicated that a high-
carbohydrate In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may o ...
diet promoted a shorter onset to sleep and a longer duration of sleep than a high-fat diet. A 2012 investigation indicated that mixed
micronutrient Micronutrients are essential dietary elements required by organisms in varying quantities throughout life to orchestrate a range of physiological functions to maintain health. Micronutrient requirements differ between organisms; for example, huma ...
s and
macronutrient A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excr ...
s are needed to promote quality sleep. A varied diet containing fresh fruits and vegetables, low
saturated fat A saturated fat is a type of fat in which the fatty acid chains have all single bonds. A fat known as a glyceride is made of two kinds of smaller molecules: a short glycerol backbone and fatty acids that each contain a long linear or branche ...
, and
whole grain A whole grain is a grain of any cereal and pseudocereal that contains the endosperm, germ, and bran, in contrast to refined grains, which retain only the endosperm. As part of a general healthy diet, consumption of whole grains is associated ...
s may be optimal for individuals seeking to improve sleep quality. High-quality
clinical trial Clinical trials are prospective biomedical or behavioral research studies on human participants designed to answer specific questions about biomedical or behavioral interventions, including new treatments (such as novel vaccines, drugs, diet ...
s on long-term dietary practices are needed to better define the influence of diet on sleep quality.


In culture


Anthropology

Research suggests that sleep patterns vary significantly across cultures. The most striking differences are observed between societies that have plentiful sources of artificial light and ones that do not. The primary difference appears to be that pre-light cultures have more broken-up sleep patterns. For example, people without artificial light might go to sleep far sooner after the sun sets, but then wake up several times throughout the night, punctuating their sleep with periods of wakefulness, perhaps lasting several hours. During
pre-industrial Pre-industrial society refers to social attributes and forums of political and cultural organization that were prevalent before the advent of the Industrial Revolution, which occurred from 1750 to 1850. ''Pre-industrial'' refers to a time befor ...
Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...
, biphasic (bimodal) sleeping was considered the norm. Sleep onset was determined not by a set bedtime, but by whether there were things to do. The boundaries between sleeping and waking are blurred in these societies. Some observers believe that nighttime sleep in these societies is most often split into two main periods, the first characterized primarily by deep sleep and the second by REM sleep. Some societies display a fragmented sleep pattern in which people sleep at all times of the day and night for shorter periods. In many
nomad A nomad is a member of a community without fixed habitation who regularly moves to and from the same areas. Such groups include hunter-gatherers, pastoral nomads (owning livestock), tinkers and trader nomads. In the twentieth century, the po ...
ic or
hunter-gatherer A traditional hunter-gatherer or forager is a human living an ancestrally derived lifestyle in which most or all food is obtained by foraging, that is, by gathering food from local sources, especially edible wild plants but also insects, fung ...
societies, people sleep on and off throughout the day or night depending on what is happening. Plentiful
artificial light Lighting or illumination is the deliberate use of light to achieve practical or aesthetic effects. Lighting includes the use of both artificial light sources like lamps and light fixtures, as well as natural illumination by capturing daylig ...
has been available in the industrialized West since at least the mid-19th century, and sleep patterns have changed significantly everywhere that lighting has been introduced. In general, people sleep in a more concentrated burst through the night, going to sleep much later, although this is not always the case. Historian A. Roger Ekirch thinks that the traditional pattern of "
segmented sleep Biphasic sleep (or ''diphasic'', ''bifurcated'', or bimodal sleep) is the practice of sleeping during two periods over the course of 24 hours, while polyphasic sleep refers to sleeping multiple times—usually more than two. Each of these is in ...
," as it is called, began to disappear among the urban upper class in Europe in the late 17th century and the change spread over the next 200 years; by the 1920s "the idea of a first and second sleep had receded entirely from our social consciousness." Ekirch attributes the change to increases in "street lighting, domestic lighting and a surge in coffee houses," which slowly made nighttime a legitimate time for activity, decreasing the time available for rest. Today in most societies people sleep during the night, but in very hot climates they may sleep during the day. During
Ramadan , type = islam , longtype = Religious , image = Ramadan montage.jpg , caption=From top, left to right: A crescent moon over Sarıçam, Turkey, marking the beginning of the Islamic month of Ramadan. Ramadan Quran reading in Bandar Torkaman, Iran. ...
, many Muslims sleep during the day rather than at night. In some societies, people sleep with at least one other person (sometimes many) or with animals. In other cultures, people rarely sleep with anyone except for an intimate partner. In almost all societies, sleeping partners are strongly regulated by social standards. For example, a person might only sleep with the
immediate family The immediate family is a defined group of relations, used in rules or laws to determine which members of a person's family are affected by those rules. It normally includes a person's parents, siblings, spouse, and children. It can contain other ...
, the
extended family An extended family is a family that extends beyond the nuclear family of parents and their children to include aunts, uncles, grandparents, cousins or other relatives, all living nearby or in the same household. Particular forms include the stem ...
, a spouse or romantic partner, children, children of a certain age, children of a specific gender, peers of a certain gender, friends, peers of equal social rank, or with no one at all. Sleep may be an actively social time, depending on the sleep groupings, with no constraints on noise or activity. People sleep in a variety of locations. Some sleep directly on the ground; others on a skin or blanket; others sleep on platforms or
bed A bed is an item of furniture that is used as a place to sleep, rest, and relax. Most modern beds consist of a soft, cushioned mattress on a bed frame. The mattress rests either on a solid base, often wood slats, or a sprung base. Many beds ...
s. Some sleep with blankets, some with pillows, some with simple headrests, some with no head support. These choices are shaped by a variety of factors, such as climate, protection from predators, housing type, technology, personal preference, and the incidence of pests.


In mythology and literature

Sleep has been seen in culture as similar to death since antiquity; in
Greek mythology A major branch of classical mythology, Greek mythology is the body of myths originally told by the ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the origin and nature of the world, the lives and activities o ...
,
Hypnos In Greek mythology, Hypnos (; Ancient Greek: means 'sleep') also spelled Hypnus is the personification of sleep; the Roman equivalent is known as Somnus. His name is the origin of the word hypnosis. Pausanias wrote that Hypnos was a dear ...
(the god of sleep) and
Thanatos In Greek mythology, Thanatos (; grc, Θάνατος, pronounced in "Death", from θνῄσκω ''thnēskō'' "(I) die, am dying") was the personification of death. He was a minor figure in Greek mythology, often referred to but rarely appea ...
(the god of death) were both said to be the children of
Nyx Nyx (; , , "Night") is the Greek goddess and personification of night. A shadowy figure, Nyx stood at or near the beginning of creation and mothered other personified deities, such as Hypnos (Sleep) and Thanatos (Death), with Erebus (Darkn ...
(the goddess of night).
John Donne John Donne ( ; 22 January 1572 – 31 March 1631) was an English poet, scholar, soldier and secretary born into a recusant family, who later became a cleric in the Church of England. Under royal patronage, he was made Dean of St Paul's Cathe ...
,
Samuel Taylor Coleridge Samuel Taylor Coleridge (; 21 October 177225 July 1834) was an English poet, literary critic, philosopher, and theologian who, with his friend William Wordsworth, was a founder of the Romantic Movement in England and a member of the Lak ...
,
Percy Bysshe Shelley Percy Bysshe Shelley ( ; 4 August 17928 July 1822) was one of the major English Romantic poets. A radical in his poetry as well as in his political and social views, Shelley did not achieve fame during his lifetime, but recognition of his achi ...
, John Keats and other poets have all written poems about the relationship between sleep and death. Shelley describes them as "both so passing, strange and wonderful!" Keats similarly poses the question: "Can death be sleep, when life is but a dream". Many people consider dying in one's sleep is the most peaceful way to die. Phrases such as "big sleep" and "
rest in peace Rest in peace (RIP), a phrase from the Latin (), is sometimes used in traditional Christian services and prayers, such as in the Catholic, Lutheran, Anglican, and Methodist denominations, to wish the soul of a decedent eternal rest and peace. ...
" are often used in reference to death, possibly in an effort to lessen its finality. Sleep and dreaming have sometimes been seen as providing the potential for visionary experiences. In medieval Irish tradition, in order to become a filí, the poet was required to undergo a ritual called the ''
imbas forosnai Imbas forosnai, is a gift of clairvoyance or visionary ability practised by the gifted poets of ancient Ireland. In Old Irish, ''Imbas'' imeans "inspiration," and specifically refers to the sacred poetic inspiration believed to be possessed b ...
'', in which they would enter a mantic, trancelike sleep. Many cultural stories have been told about people falling asleep for extended periods of time. The earliest of these stories is the ancient Greek legend of Epimenides of Knossos. According to the biographer
Diogenes Laërtius Diogenes Laërtius ( ; grc-gre, Διογένης Λαέρτιος, ; ) was a biographer of the Greek philosophers. Nothing is definitively known about his life, but his surviving ''Lives and Opinions of Eminent Philosophers'' is a principal sour ...
, Epimenides was a shepherd on the Greek island of
Crete Crete ( el, Κρήτη, translit=, Modern: , Ancient: ) is the largest and most populous of the Greek islands, the 88th largest island in the world and the fifth largest island in the Mediterranean Sea, after Sicily, Sardinia, Cyprus, ...
. One day, one of his sheep went missing and he went out to look for it, but became tired and fell asleep in a cave under
Mount Ida In Greek mythology, two sacred mountains are called Mount Ida, the "Mountain of the Goddess": Mount Ida (Crete), Mount Ida in Crete, and Mount Ida (Turkey), Mount Ida in the ancient Troad region of western Anatolia (in modern-day Turkey), which wa ...
. When he awoke, he continued searching for the sheep, but could not find it, so he returned to his old farm, only to discover that it was now under new ownership. He went to his hometown, but discovered that nobody there knew him. Finally, he met his younger brother, who was now an old man, and learned that he had been asleep in the cave for fifty-seven years. A far more famous instance of a "long sleep" today is the Christian
legend A legend is a genre of folklore that consists of a narrative featuring human actions, believed or perceived, both by teller and listeners, to have taken place in human history. Narratives in this genre may demonstrate human values, and possess ...
of the Seven Sleepers of Ephesus, in which seven Christians flee into a cave during pagan times in order to escape
persecution Persecution is the systematic mistreatment of an individual or group by another individual or group. The most common forms are religious persecution, racism, and political persecution, though there is naturally some overlap between these ter ...
, but fall asleep and wake up 360 years later to discover, to their astonishment, that the Roman Empire is now predominantly Christian. The American author
Washington Irving Washington Irving (April 3, 1783 – November 28, 1859) was an American short-story writer, essayist, biographer, historian, and diplomat of the early 19th century. He is best known for his short stories "Rip Van Winkle" (1819) and " The Legen ...
's short story "
Rip Van Winkle "Rip Van Winkle" is a short story by the American author Washington Irving, first published in 1819. It follows a Dutch-American villager in colonial America named Rip Van Winkle who meets mysterious Dutchmen, imbibes their liquor and falls aslee ...
", first published in 1819 in his collection of short stories '' The Sketch Book of Geoffrey Crayon, Gent.'', is about a man in
colonial America The colonial history of the United States covers the history of European colonization of North America from the early 17th century until the incorporation of the Thirteen Colonies into the United States after the Revolutionary War. In the ...
named Rip Van Winkle who falls asleep on one of the
Catskill Mountains The Catskill Mountains, also known as the Catskills, are a physiographic province of the larger Appalachian Mountains, located in southeastern New York. As a cultural and geographic region, the Catskills are generally defined as those areas cl ...
and wakes up twenty years later after the
American Revolution The American Revolution was an ideological and political revolution that occurred in British America between 1765 and 1791. The Americans in the Thirteen Colonies formed independent states that defeated the British in the American Revoluti ...
. The story is now considered one of the greatest classics of
American literature American literature is literature written or produced in the United States of America and in the colonies that preceded it. The American literary tradition thus is part of the broader tradition of English-language literature, but also inc ...
.


In art

Of the thematic representations of sleep in art, physician and sleep researcher Meir Kryger wrote, " rtistshave intense fascination with mythology, dreams, religious themes, the parallel between sleep and death, reward, abandonment of conscious control, healing, a depiction of innocence and serenity, and the erotic." File:Carel Fabritius - De poort bewaker (1654).jpg, ''
The Sentry Sentry or The Sentry may refer to: Comics *Sentry (Kree) * Sentry (Curtis Elkins) *Sentry (Robert Reynolds) * Senator Ward (comics) or Sentry Vehicles * Sentry (AUV), an autonomous underwater vehicle used to measure deep-ocean data *E-3 Sentry AW ...
'' (1654) by
Carel Fabritius Carel Pietersz. Fabritius (; bapt. 27 February 1622 – 12 October 1654) was a Dutch painter. He was a pupil of Rembrandt and worked in his studio in Amsterdam. Fabritius, who was a member of the Delft School, developed his own artistic style ...
File:Museo del Prado - Goya - Caprichos - No. 43 - El sueño de la razon produce monstruos.jpg, ''
The Sleep of Reason Produces Monsters ''The Sleep of Reason Produces Monsters'' or ''The Dream of Reason Produces Monsters'' ( es, El sueño de la razón produce monstruos) is an aquatint by the Spanish painter and printmaker Francisco Goya. Created between 1797 and 1799 for the , it ...
'' (1799) by
Francisco Goya Francisco José de Goya y Lucientes (; ; 30 March 174616 April 1828) was a Spanish Romanticism, romantic painter and Printmaking, printmaker. He is considered the most important Spanish artist of the late 18th and early 19th centuries. His p ...
File:Honoré Daumier - The Second Class Carriage - Walters 371224.jpg, ''The Second Class Carriage'' (1864) by
Honoré Daumier Honoré-Victorin Daumier (; February 26, 1808February 10, 1879) was a French painter, sculptor, and printmaker, whose many works offer commentary on the social and political life in France, from the Revolution of 1830 to the fall of the second N ...
File:Waterhouse-sleep and his half-brother death-1874.jpg, '' Sleep and his Half-brother Death'' (1874) by
John William Waterhouse John William Waterhouse (6 April 184910 February 1917) was an English painter known for working first in the Academic style and for then embracing the Pre-Raphaelite Brotherhood's style and subject matter. His artworks were known for their de ...
File:William-Adolphe Bouguereau (1825-1905) - Lullaby (1875).jpg, ''Lullaby'' (1875) by
William-Adolphe Bouguereau William-Adolphe Bouguereau (; 30 November 1825 – 19 August 1905) was a French academic painter. In his realistic genre paintings, he used mythological themes, making modern interpretations of classical subjects, with an emphasis on the female ...
File:Илья Е. Репин - Отдых. Портрет В.А.Репинa (1882).jpg, ''Taking a Rest'' (1882) by
Ilya Repin Ilya Yefimovich Repin (russian: Илья Ефимович Репин, translit=Il'ya Yefimovich Repin, p=ˈrʲepʲɪn); fi, Ilja Jefimovitš Repin ( – 29 September 1930) was a Russian painter, born in what is now Ukraine. He became one of the ...
File:George Hare - Victory of Faith.jpg, '' The Victory of Faith'' (1891) by
Saint George Hare Saint George Hare (5 July 1857 Limerick – London, 1933) was an Irish artist. Life He was the son of George Frederick Hare, a dentist from Ipswich, and his wife, Ella, from County Wexford. He was formally educated in art in Limerick Schoo ...
File:Auf dem Ofen 1895.jpg, ''Zwei schlafende Mädchen auf der Ofenbank'' (1895) by
Albert Anker Albrecht Samuel Anker (April 1, 1831 – July 16, 1910) was a Swiss painter and illustrator who has been called the "national painter" of Switzerland because of his enduringly popular depictions of 19th-century Swiss village life. Life Bor ...
File:Flaming June, by Frederic Lord Leighton (1830-1896).jpg, ''
Flaming June ''Flaming June'' is a painting by Sir Frederic Leighton, produced in 1895. Painted with oil paints on a square canvas, it is widely considered to be Leighton's magnum opus, showing his classicist nature. It is thought that the woman portrayed ...
'' ( 1895) by
Frederic Leighton Frederic Leighton, 1st Baron Leighton, (3 December 1830 – 25 January 1896), known as Sir Frederic Leighton between 1878 and 1896, was a British painter, draughtsman, and sculptor. His works depicted historical, biblical, and classical subjec ...
File:Noon, rest from work - Van Gogh.jpeg, ''Noon – Rest from Work'' (1890) by
Vincent van Gogh Vincent Willem van Gogh (; 30 March 185329 July 1890) was a Dutch Post-Impressionist painter who posthumously became one of the most famous and influential figures in Western art history. In a decade, he created about 2,100 artworks, inc ...
(after
Millet Millets () are a highly varied group of small-seeded grasses, widely grown around the world as cereal crops or grains for fodder and human food. Most species generally referred to as millets belong to the tribe Paniceae, but some millets a ...
) File:Albert Anker - Schlafendes Mädchen auf einer Holzbank.jpg, ''Sleeping Girl on a Wooden Bench'' by
Albert Anker Albrecht Samuel Anker (April 1, 1831 – July 16, 1910) was a Swiss painter and illustrator who has been called the "national painter" of Switzerland because of his enduringly popular depictions of 19th-century Swiss village life. Life Bor ...
File:Carlos Maria Herrera - Descansando.jpg, ''Descansando'' (1900) by Carlos Maria Herrera


See also

*
Co-sleeping Co-sleeping is a practice in which babies and young children sleep close to one or both parents, as opposed to in a separate room. Co-sleeping individuals sleep in sensory proximity to one another, where the individual senses the presence of othe ...
*
Hypnogram A hypnogram is a form of polysomnography; it is a graph that represents the stages of sleep as a function of time. It was developed as an easy way to present the recordings of the brain wave activity from an electroencephalogram (EEG) during a perio ...
*
Microsleep A microsleep is a sudden temporary episode of sleep or drowsiness which may last for a few seconds where an individual fails to respond to some arbitrary sensory input and becomes unconscious.International Classification of Sleep Disorders, , pa ...
*
Morvan's syndrome Morvan's syndrome is a rare, life-threatening autoimmune disease named after the nineteenth century French physician Augustin Marie Morvan. "La chorée fibrillaire" was first coined by Morvan in 1890 when describing patients with multiple, irregu ...
*
Oleamide Oleamide is an organic compound with the formula CH3(CH2)7CH=CH(CH2)7CONH2(. It is the amide derived from the fatty acid oleic acid. It is a colorless waxy solid and occurs in nature. Sometimes labeled as a fatty acid primary amide (FAPA), it ...
* Power nap *
Rheum Rheum (; from Greek: ῥεῦμα ''rheuma'' 'a flowing, rheum') is a thin mucus naturally discharged from the eyes, nose, or mouth, often during sleep (cf. mucopurulent discharge).Amodio, Aime"Where Do Eye Boogers Come From?" Families.com bl ...
* Sleep epidemiology *
Sleep in space Sleeping in space is an important part of space medicine and mission planning, with impacts on the health, capabilities and morale of astronauts. Human spaceflight often requires astronaut crews to endure long periods without rest. Studies hav ...
* Sleep-learning *
Sleep medicine Sleep medicine is a medical specialty or subspecialty devoted to the diagnosis and therapy of sleep disturbances and disorders. From the middle of the 20th century, research has provided increasing knowledge and answered many questions about ...
* Sleep paralysis * Sleeping positions *
Somnology Somnology is the scientific study of sleep. It includes clinical study and treatment of sleep disorders and irregularities. Sleep medicine is a subset of somnology. History After the invention of the EEG, the stages of sleep were determined in 1 ...
* Somnophilia * Start school later movement *
Sudden infant death syndrome Sudden infant death syndrome (SIDS) is the sudden unexplained death of a child of less than one year of age. Diagnosis requires that the death remain unexplained even after a thorough autopsy and detailed death scene investigation. SIDS usuall ...
* Sudden arrhythmic death syndrome *
Unconsciousness Unconsciousness is a state in which a living individual exhibits a complete, or near-complete, inability to maintain an awareness of self and environment or to respond to any human or environmental stimulus. Unconsciousness may occur as the r ...
*
Yawn A yawn is a reflex lasting 4-7 seconds, and is characterized by a long inspiratory phase with gradual mouth gaping, followed by a brief climax (or acme) with muscle stretching, and a rapid expiratory phase with muscle relaxation. For fish an ...
*
Yoga-nidra Yoga nidra ( sa, योग निद्रा, ) or yogic sleep in modern usage is a state of consciousness between waking and sleeping, typically induced by a guided meditation. A state called yoga nidra is mentioned in the Upanishads and the ...


References


Further reading

*


External links


Rethinking Sleep
David K. Randall, ''New York Times,'' September 2012
How to Sleep
James Hamblin, ''The Atlantic,'' January 2017 {{Authority control Mental states Unsolved problems in neuroscience