biological clock

The biological clock is the body's internal timekeeper — the machinery that generates daily rhythms and keeps physiological events scheduled to the right time of day. Like a watch that keeps running even when you cannot see the sun, it ticks on its own, so the body is not simply reacting to light but actively anticipating the cycle of day and night.

At the molecular level the clock is a self-sustaining loop of clock genes whose proteins build up, switch off their own production, get degraded, and start again over roughly a day. These molecular oscillators run in cells throughout the body, but they are coordinated by a master clock in the brain so that all the peripheral clocks keep the same time.

The clock is what schedules circadian hormone rhythms — when cortisol peaks, when melatonin rises, when growth hormone surges. It runs slightly off from exactly 24 hours, so it must be reset, or entrained, each day by external cues, mainly light. When entrainment fails or the clock is forced out of step, as in shift work, hormone timing and metabolism suffer.