Circadian rhythm

An internally generated biological cycle with a period of about 24 hours that can synchronize to environmental cues such as light and darkness.

Circadian clock: An endogenous timing system that organizes biological processes into recurring daily patterns. The rhythm is the recurring pattern produced and organized by this internal timing system.
Free-running rhythm: An endogenous rhythm that continues without periodic environmental cues. Persistence without cues shows that the cycle is generated internally.
Chronotype: An individual's characteristic timing preference for sleep and activity relative to the day-night cycle. Different chronotypes reflect variation in the phase of daily timing.
Ultradian rhythm: A biological rhythm that repeats more than once within a 24-hour period. Its shorter period contrasts with the approximately daily cycle.
Suprachiasmatic nucleus: A hypothalamic brain region that coordinates mammalian circadian timing, especially in response to light. It receives light-related signals and coordinates daily rhythms across the body.
Circadian period: The duration of one complete cycle of an endogenous circadian rhythm. Its near-24-hour length is the defining timing feature of the rhythm.
Jet lag: A temporary mismatch between internal circadian timing and the local time zone after rapid travel. Rapid time-zone changes leave internal rhythms out of step with local time.
Infradian rhythm: A biological rhythm whose period is longer than 24 hours. Its longer cycle separates it from circadian timing.
CLOCK gene: A gene encoding a transcription factor that helps regulate molecular circadian feedback loops. Its protein participates in the feedback machinery that generates daily timing.
Phase response curve: A graph showing how a stimulus shifts the timing of an oscillating biological system at different phases. It predicts whether a cue advances or delays the rhythm depending on its timing.

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