The circadian rhythm is the body's roughly 24-hour internal clock. It controls the sleep-wake cycle and the daily timing of many hormones, body temperature, and metabolism, and is set largely by light and dark.
It governs the normal daily pattern of cortisol, which typically rises to a peak shortly after waking and falls to its lowest around bedtime. Disrupted rhythms — from shift work, jet lag, or poor sleep habits — can throw cortisol and sleep out of sync; see cortisol and sleep.