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What is the circadian rhythm?

Medicine & HealthLesson on requestFree to read

Quick answer

The circadian rhythm is your body's roughly 24-hour internal clock that controls sleep, alertness, hormone release, and other functions. It is set mainly by light and dark, telling you when to feel awake during the day and sleepy at night.

Key things to understand

  1. A master clock in the brain (the suprachiasmatic nucleus) keeps the body on a ~24-hour cycle.
  2. Light is the strongest signal that resets it — bright morning light advances it, late-night light delays it.
  3. It governs the timing of hormones like melatonin (sleep) and cortisol (alertness).
  4. Disrupting it through jet lag, shift work, or late screens harms sleep, mood, and health.
  5. Keeping consistent light and sleep times keeps the rhythm well-aligned.

People also ask

What controls the circadian rhythm?
A cluster of brain cells acts as a master clock, synced mainly by light entering the eyes, which is why daylight and darkness shape your sleep timing.
Why does jet lag happen?
Crossing time zones puts your internal clock out of step with local light and time, so you feel sleepy or alert at the wrong hours until it resets.
How do screens affect it?
Blue-rich light at night signals 'daytime' to the clock, suppressing melatonin and making it harder to fall asleep.

Part of the guide: How Your Body Works →