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SleepUncovered

Circadian rhythm, explained simply

Updated16 May 2026Read time6 minReviewed bySleepUncovered editorial

Short answer

Strong evidence

Your circadian rhythm is an internal clock — actually a set of clocks — that runs on roughly a 24-hour cycle. It governs when you feel sleepy, when you feel alert, when hormones release, and when your body temperature peaks and falls. It's set primarily by light exposure, with food timing and exercise as weaker secondary signals.

Key points

  • The master clock is the suprachiasmatic nucleus (SCN), a tiny cluster of neurons in the hypothalamus that receives light signals from the eyes.
  • Your endogenous rhythm is closer to 24.2 hours, not exactly 24. Daily light exposure 'resets' it; without light, your rhythm drifts.
  • Light in the morning advances your clock (makes you a morning person). Light in the evening delays it (makes you a night owl).
  • Every cell in your body has its own peripheral clock, synced from the SCN. They control thousands of genes in a daily rhythm.
  • Misalignment between your circadian rhythm and your schedule is called social jetlag — and it has measurable metabolic and mood consequences.

The two-process model

Sleep is governed by two systems running in parallel:

  • Process S— sleep pressure. Builds the longer you're awake (driven by adenosine accumulation), dissipates while you sleep. Linear, like filling and draining a tank.
  • Process C— the circadian rhythm. A roughly 24-hour oscillation, independent of how long you've been awake. Says “it's sleep time” or “it's alert time” based on the clock alone.

You sleep well when both align. You sleep badly when they don't — for example, when you're desperately tired (high sleep pressure) but your circadian rhythm is screaming “alert!” (jet lag eastward, or trying to sleep at 2pm after a night shift).

The master clock — the SCN

Deep in the hypothalamus sits the suprachiasmatic nucleus (SCN), roughly 20,000 neurons that act as the body's master timekeeper. Light hitting specialised retinal ganglion cells signals directly to the SCN. The SCN then synchronises hormone release, body temperature, and the thousands of peripheral clocks in other tissues.

These peripheral clocks are not optional. Roughly 10% of all gene expression in any given cell follows a daily rhythm. The liver expresses different genes at 3am than at 3pm.

Light is the strongest input

Light exposure shifts the circadian clock in opposite directions depending on timing:

  • Morning light advances the clock — pulls everything earlier. Wake earlier, get sleepy earlier.
  • Evening light delays it — pushes everything later. Stay up later, sleep in later.
  • Midday light has essentially no shifting effect, though it still reinforces the existing rhythm.

This is why getting outside in the morning is the most powerful intervention for evening chronotypes — and why scrolling on a bright phone at midnight tends to make you a night owl over time. See light exposure for practical detail.

Other inputs

Light is dominant, but not the only zeitgeber:

  • Meal timing sets peripheral clocks in the liver and gut. Eating at consistent times reinforces a stable rhythm.
  • Exercise is a mild zeitgeber, with effects depending on time of day.
  • Social cues — meetings, social interaction — are weak but real signals.

Social jetlag

Most adults sleep on different schedules during the week and at weekends. The difference between your weekday and weekend midsleep is called social jetlag, and people with more than two hours of it have worse metabolic markers, mood, and cognitive performance in observational data (Roenneberg et al, 2012). It's the same biological mechanism as flying east and west repeatedly.

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