Circadian Rhythm
The body’s internal, roughly-24-hour clock — the regulatory system that times daily cycles of sleep, alertness, hormone release, body temperature, and metabolism.
The master clock
Section titled “The master clock”A small region of the hypothalamus (the suprachiasmatic nucleus) is the body’s master clock. It runs on its own near-24-hour cycle and is reset each day by light reaching the eyes — keeping it synchronized with the actual day.
How it is broadcast
Section titled “How it is broadcast”The master clock signals the rest of the body partly through melatonin, released by the pineal gland in darkness. Most organs also keep their own local clocks, all kept in step with the master.
When it is disrupted
Section titled “When it is disrupted”Jet lag and shift work pull the internal clock out of step with the environment, degrading sleep, mood, and metabolic health.
Key idea
Section titled “Key idea”Circadian rhythm is regulation in the dimension of time — the body anticipates the coming day rather than merely reacting to it.