circadian rhythm

A circadian rhythm is an internal cycle that repeats roughly every 24 hours, like an inner clock that keeps ticking even in a cave with no daylight. The body uses it to anticipate the day-night cycle, pre-tuning hormone levels, body temperature, sleep, and metabolism for the part of the day that is coming rather than just reacting to it.

Many hormones follow strong circadian patterns. Cortisol peaks in the early morning to help you wake and mobilize energy, then falls to a low at night; melatonin does the opposite, rising in darkness to promote sleep; growth hormone surges in early deep sleep. These rhythms are generated internally — the term circadian, from Latin for about a day, signals that they persist near 24 hours even without external cues.

Because the rhythm is endogenous but normally synchronized to the light-dark cycle, it can be thrown off when the two disagree. Jet lag and night-shift work misalign the internal clock with the outside world, disrupting cortisol and melatonin timing; chronic misalignment is linked to metabolic and sleep disturbances.