A consistent pattern of cyclical body activities, usually lasting 24 to 25 hours and determined by an internal biological clock.

innate daily fluctuation of physiological or behavioral functions, including sleep-wake states generally tied to the 24-hour daily dark-light cycle

A pattern of behavioral, biochemical, or physiological fluctuation that has a 24-hour period.

a physiological cycle of about 24 hours that is present in all eukaryotic organisms and that persists even in the absence of external cues

The body's biological clock that regulates the sleep/wake cycle and other physiological processes.

innate, daily, fluctuation of behavioral and physiological functions, including sleep waking, generally tied to the 24 hour day-night cycle but sometimes to a different (e.g., 23 or 25 hour) periodici [..]

The circadian rhythm or body clock is a biological process that occurs naturally on a 24-hour cycle (i.e. a full day). The word ‘circadian’ comes from the Latin circa (around) and dies (days).

Biological rhythms that repeat approximately every 24 hours are called circadian rhythms. Light helps maintain the circadian clock, and researchers are working to better understand how lighting can support or disrupt circadian rhythms.

The regular Recurrence, in cycles of about 24 hours, of Biological Processes or activities, such as sensitivity to Drugs and stimuli, Hormone Secretion, sleeping, and feeding.

The regular recurrence, in cycles of about 24 hours, of biological processes or activities, such as sensitivity to drugs and stimuli, hormone secretion, sleeping, feeding, etc. This rhythm seems to be [..]

 A cycle of behavior or physiological change lasting approximately 24 hours. 

An endogenous rhythm involving a response at about 24-hour intervals. Circulatory virus:

A cycle of behavior or physiological change lasting approximately 24 hours.

The "internal body clock" that regulates the (roughly) 24-hour cycle of biological processes in animals and plants.