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Biological rhythms are repetitive
biological process Biological processes are those processes that are vital for an organism to live, and that shape its capacities for interacting with its environment. Biological processes are made of many chemical reactions or other events that are involved in the ...
es. Some types of biological rhythms have been described as biological clocks. They can range in frequency from microseconds to less than one repetitive event per decade. Biological rhythms are studied by
chronobiology Chronobiology is a field of biology that examines timing processes, including periodic (cyclic) phenomena in living organisms, such as their adaptation to solar- and lunar-related rhythms. These cycles are known as biological rhythms. Chronob ...
. In the biochemical context biological rhythms are called biochemical oscillations. The variations of the timing and duration of biological activity in living organisms occur for many essential biological processes. These occur (a) in animals (eating, sleeping, mating, hibernating, migration, cellular regeneration, etc.), (b) in plants (leaf movements, photosynthetic reactions, etc.), and in microbial organisms such as fungi and protozoa. They have even been found in bacteria, especially among the cyanobacteria (aka blue-green algae, see
bacterial circadian rhythms Bacterial circadian rhythms, like other circadian rhythms, are endogenous "biological clocks" that have the following three characteristics: (a) in constant conditions (i.e. constant temperature and either constant light or constant darkness ) they ...
).


Circadian rhythm

The best studied rhythm in chronobiology is the
circadian rhythm 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 ...
, a roughly 24-hour cycle shown by physiological processes in all these organisms. The term ''circadian'' comes from the Latin ''circa'', meaning "around" and ''dies'', "day", meaning "approximately a day." It is regulated by
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). ...
s. The circadian rhythm can further be broken down into routine cycles during the 24-hour day: * Diurnal, which describes organisms active during daytime *
Nocturnal Nocturnality is an animal behavior characterized by being active during the night and sleeping during the day. The common adjective is "nocturnal", versus diurnal meaning the opposite. Nocturnal creatures generally have highly developed sens ...
, which describes organisms active in the night *
Crepuscular In zoology, a crepuscular animal is one that is active primarily during the twilight period, being matutinal, vespertine, or both. This is distinguished from diurnal and nocturnal behavior, where an animal is active during the hours of daylig ...
, which describes animals primarily active during the dawn and dusk hours (ex: white-tailed deer, some bats) While
circadian rhythms 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., Endogeny (biology), endogeno ...
are defined as regulated by endogenous processes, other biological cycles may be regulated by exogenous signals. In some cases, multi-trophic systems may exhibit rhythms driven by the circadian clock of one of the members (which may also be influenced or reset by external factors). The endogenous plant cycles may regulate the activity of the bacterium by controlling availability of plant-produced photosynthate.


Other cycles

Many other important cycles are also studied, including: *
Infradian rhythm In chronobiology, an infradian rhythm is a rhythm with a period longer than the period of a circadian rhythm, i.e., with a frequency of less than one cycle in 24 hours. Some examples of infradian rhythms in mammals include menstruation, breeding, m ...
s, which are cycles longer than a day. Examples include circannual or annual cycles that govern migration or reproduction cycles in many plants and animals, or the human menstrual cycle. *
Ultradian rhythm In chronobiology, an ultradian rhythm is a recurrent period or cycle repeated throughout a 24-hour day. In contrast, circadian rhythms complete one cycle daily, while infradian rhythms such as the human menstrual cycle have periods longer than ...
s, which are cycles shorter than 24 hours, such as the 90-minute
REM cycle Rapid eye movement sleep (REM sleep or REMS) is a unique phase of sleep in mammals and birds, characterized by random rapid movement of the eyes, accompanied by low muscle tone throughout the body, and the propensity of the sleeper to dream viv ...
, the 4-hour
nasal cycle The nasal cycle is the unconscious alternating partial congestion and decongestion of the nasal cavities in humans and other animals. This results in greater airflow through one nostril with periodic alternation between the nostrils. It is a phy ...
, or the 3-hour cycle of growth hormone production. * Tidal rhythms, commonly observed in marine life, which follow the roughly 12.4-hour transition from high to low tide and back. *
Lunar rhythms The lunar effect is a purported unproven correlation between specific stages of the roughly 29.5-day lunar cycle and behavior and physiological changes in living beings on Earth, including humans. In some cases the purported effect may depend on ...
, which follow the
lunar month In lunar calendars, a lunar month is the time between two successive syzygies of the same type: new moons or full moons. The precise definition varies, especially for the beginning of the month. Variations In Shona, Middle Eastern, and Eur ...
(29.5 days). They are relevant e.g. for marine life, as the level of the tides is modulated across the lunar cycle. *
Gene oscillations In molecular biology, an oscillating gene is a gene that is expressed in a rhythmic pattern or in periodic cycles. Oscillating genes are usually circadian and can be identified by periodic changes in the state of an organism. Circadian rhythms, c ...
– some genes are expressed more during certain hours of the day than during other hours. Within each cycle, the time period during which the process is more active is called the '' acrophase''.Refinetti, Roberto (2006). ''Circadian Physiology''. CRC Press/Taylor & Francis Group. .
Lay summary
/ref> When the process is less active, the cycle is in its ''
bathyphase Chronobiology is a field of biology that examines timing processes, including periodic (cyclic) phenomena in living organisms, such as their adaptation to solar- and lunar-related rhythms. These cycles are known as biological rhythms. Chronob ...
'' or ''trough'' phase. The particular moment of highest activity is the ''peak'' or ''maximum''; the lowest point is the ''nadir''. How high (or low) the process gets is measured by the ''
amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of ampl ...
''.


Biochemical basis of biological rhythms

Goldbeter's book provides a thorough analysis of the biochemical mechanisms and their kinetic properties that underlie biological rhythms.


References


External links


Society for Research on Biological Rhythms

Biological rhythm
in Encyclopedia Britannica {{Authority control * Biological processes