Dictionary of Human Evolution and Biology

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Circadian Rhythm

Cycles of metabolic activity and behavior that occur within a 24 h pattern. This daily rhythmicity includes hunger, wakefulness, and sleeping. Some circadian rhythms are influenced by the cycle of light and darkness throughout the year; however, when humans or animals are isolated from the outside environment circadian rhythms remain fairly intact. The mechanism that controls circadian rhythm is the endogenous biological clock.

Cf. infradian rhythm, ultradian rhythm.

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