Inherent cycle of approximately 24 hours in length that appears to control or initiate various biological processes, including sleep, wakefulness, and digestive and hormonal activity.
The natural signal for the circadian pattern is the change from darkness to light. The controlling mechanism for these cyclic processes within the body is thought to be the hypothalamus . Any change in the circadian cycle (such as jet lag and other conditions associated with travel) requires a certain period for readjustment.