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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. In most living things, internally synchronized circadian clocks make it possible for the organism to anticipate daily environmental changes corresponding with the day–night cycle and adjust its biology and behavior accordingly.
The term circadian derives from the Latin circa dies , since when taken away from external cues , 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: