Circadian Rhythm

Your 24-hour clock

How does circadian Rhythm work?

A master clock in the brain, set mainly by light, times sleep, hormones, body temperature and digestion.

How it works, step by step

1. Morning light hits special retinal cells.
2. They signal the brain's master clock (SCN).
3. The clock sets timing of cortisol, temperature and melatonin.
4. Evening darkness lets melatonin rise, promoting sleep.

Practical

Bright light in the morning and dimmer light at night help keep the clock aligned.

Evidence & sources

• Light is the main signal that keeps the body's circadian clock synchronized with the day. Evidence: established. Known: Well established in chronobiology. Unknown: — Source: NIH NIGMS, Circadian Rhythms fact sheet (Current revision).

Connected topics

• Sleep & Recovery
• Eye
• Hypothalamus
• Adrenal Glands
• Melatonin Supplements

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General education only, not medical advice. Drafted Oct 2026 · pending qualified expert review.