Descriptive of a physiological activity that occurs approximately every twenty-four hours, or the rhythm of such activity: The doctor told Raymond that his circadian rhythm referred to his cycle of activities; including both his daily functioning (eating, sleeping, etc.) and his body's chemistry (changes in blood pressure, urine production, etc.) all of which usually last for and are repeated about every 24 hours.

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 set by a "biological clock" which seems to be set by recurring daylight and darkness.