1) 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.

2) Rhythmic recurrences of certain phenomena in living organisms occurring at about the same time of day, i.e., on a 24 hour cycle.