Free-running sleep is a rare

sleep Sleep is a sedentary state of mind and body. It is characterized by altered consciousness, relatively inhibited Perception, sensory activity, reduced muscle activity and reduced interactions with surroundings. It is distinguished from wakefuln ...

pattern whereby the sleep schedule of a person shifts later every day. It occurs as the

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 tes ...

non-24-hour sleep–wake disorder or artificially as part of experiments used in the study of

circadian A circadian rhythm (), or circadian cycle, is a natural, internal process that regulates the sleep–wake cycle and repeats roughly every 24 hours. It can refer to any process that originates within an organism (i.e., endogenous) and responds to ...

and other rhythms in

biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditar ...

. Study subjects are shielded from all time cues, often by a constant light protocol, by a constant dark protocol or by the use of light/dark conditions to which the organism cannot entrain such as the ultrashort protocol of one hour dark and two hours light. Also, limited amounts of food may be made available at short intervals so as to avoid entrainment to mealtimes. Subjects are thus forced to live by their internal circadian "clocks".

Background

The individual's or animal's circadian phase can be known only by the monitoring of some kind of output of the circadian system, the internal "body clock". The researcher can precisely determine, for example, the daily cycles of

gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

activity,

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 ...

blood pressure Blood pressure (BP) is the pressure of circulating blood against the walls of blood vessels. Most of this pressure results from the heart pumping blood through the circulatory system. When used without qualification, the term "blood pressur ...

hormone 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 ...

secretion and/or

and activity/ alertness. Alertness in humans can be determined by many kinds of verbal and non-verbal tests, whereas alertness in animals can usually be assessed by observing physical activity (for example, of wheel-running in

rodent Rodents (from Latin , 'to gnaw') are mammals of the Order (biology), order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are roden ...

s). When animals or people ''free-run'', experiments can be done to see what sort of signals, known as zeitgebers, are effective in entrainment. Also, much work has been done to see how long or short a circadian cycle can be entrained to various organisms. For example, some animals can be entrained to a 22-hour day, but they can not be entrained to a 20-hour day. In recent studies funded by the U.S. space industry, it has been shown that most humans can be entrained to a 23.5-hour day and to a 24.65-hour day. The effect of unintended time cues is called ''masking'' and can totally confound experimental results. Examples of masking are morning rush traffic audible to the subjects, or researchers or maintenance staff visiting subjects on a regular schedule.

In humans

Non-24-hour sleep–wake disorder, also referred to as ''free-running disorder'' (FRD) or ''Non-24'', is one of the circadian rhythm sleep disorders in humans. It affects more than half of people who are totally

blind Blind may refer to: * The state of blindness, being unable to see * A window blind, a covering for a window Blind may also refer to: Arts, entertainment, and media Films * ''Blind'' (2007 film), a Dutch drama by Tamar van den Dop * ''Blind' ...

and a smaller number of sighted individuals.Sack RL et al. (2007) ''Circadian Rhythm Sleep Disorders: Part II, Advanced Sleep Phase Disorder, Delayed Sleep Phase Disorder, Free-Running Disorder, and Irregular Sleep–Wake Rhythm.''
PDF, 30(11):1484-1501. Among blind people, the cause is the inability to register, and therefore to entrain to, light cues. The many blind people who do entrain to the 24-hour light/dark cycle have eyes with functioning retinas including operative non-visual light-sensitive cells, ipRGCs. These ganglion cells, which contain

melanopsin Melanopsin is a type of photopigment belonging to a larger family of light-sensitive retinal proteins called opsins and encoded by the gene ''Opn4''. In the mammalian retina, there are two additional categories of opsins, both involved in the fo ...

, convey their signals to the "circadian clock" via the

retinohypothalamic tract In neuroanatomy, the retinohypothalamic tract (RHT) is a photic neural input pathway involved in the circadian rhythms of mammals. The origin of the retinohypothalamic tract is the intrinsically photosensitive retinal ganglion cells (ipRGC), ...

(branching off from the

optic nerve In neuroanatomy, the optic nerve, also known as the second cranial nerve, cranial nerve II, or simply CN II, is a paired cranial nerve that transmits visual information from the retina to the brain. In humans, the optic nerve is derived fro ...

), linking the retina 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 ...

. Among sighted individuals, Non-24 usually first appears in the teens or early twenties. As with delayed sleep phase disorder (DSPS or DSPD), in the absence of neurological damage due to trauma or stroke, cases almost never appear after the age of 30. Non-24 affects more sighted males than sighted females. A quarter of sighted individuals with Non-24 also have an associated psychiatric condition, and a quarter of them have previously shown symptoms of DSPS.

References

External links

A collection of articles about sleep
by Piotr A. Wozniak, July 2000 {{Light Ethology Sleep Circadian rhythm

Background

In humans

See also

References

External links