Circadian clock

The circadian clock, or circadian oscillator, in most living things makes it possible for organisms to coordinate their biology and behavior with daily environmental changes in the day-night cycle. 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. Hence the term circadian.

The normal body clock oscillates with a period of exactly 24 hours when it receives 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: