Circadian clock

A circadian clock, or circadian oscillator, is a biochemical oscillator that oscillates with a stable phase relationship to solar time. Such a clock's in vivo period, averaged over an earth year, is necessarily almost exactly 24 hours (the earth's current solar day). In most living things, internally synchronized circadian clocks make it possible for the organism to coordinate its biology and behavior with daily environmental changes corresponding with the day–night cycle and derived diurnal behaviour patterns (e.g. crepuscular feeding).

The term circadian derives from the Latin circa (about) diem (a day), since when taken away from external cues (such as the day-night cycle), they do not run to exactly 24 hours. Clocks in humans in a lab in constant low light, for example, will average about 24.2 hours per day, rather than 24 hours exactly.

The normal body clock oscillates with an endogenous period of exactly 24 hours, it entrains, when it receives sufficient daily corrective signals from the environment, primarily daylight and darkness. Circadian clocks are the central mechanisms that drive circadian rhythms. They consist of three major components:

The clock is reset as an organism senses environmental time cues of which the primary one is light. Circadian oscillators are ubiquitous in tissues of the body where they are synchronized by both endogenous and external signals to regulate transcriptional activity throughout the day in a tissue-specific manner. The circadian clock is intertwined with most cellular metabolic processes and it is affected by organism aging. The basic molecular mechanisms of the biological clock have been defined in vertebrate species, Drosophila melanogaster, plants, fungi, bacteria, and presumably also in Archaea.

While a precise 24-hour circadian clock is found in many organisms, it is not universal. Organisms living in the high arctic or high antarctic do not experience solar time in all seasons, though most are believed to maintain a circadian rhythm close to 24 hours, such as bears during torpor. Much of the earth's biomass resides in the dark biosphere, and while these organisms may exhibit rhythmic physiology, for these organisms the dominant rhythm is unlikely to be circadian. For east-west migratory organisms—and especially should an organism circumnavigate the globe—the absolute 24-hour phase might deviate over months, seasons, or years.

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