Sleep · lesson 4 of 11
Your circadian rhythm
The internal ~24-hour clock that decides when you can sleep well.
6 min read · reviewed October 2026
The master clock
Deep in your brain, the suprachiasmatic nucleus (SCN) runs a roughly 24-hour clock that governs when you feel sleepy and alert, your core temperature, and hormone release. Left alone it actually runs slightly long (~24.2 hours), so it has to be reset daily by external cues — above all, light.
Zeitgebers — the cues that set your clock
By far the strongest signal. Bright morning light advances the clock (earlier sleep); bright evening light delays it (later sleep).
Released by the pineal gland as darkness falls — the body's 'it's night' signal. Suppressed by light, especially blue light.
Core body temperature falls at night and bottoms out a couple of hours before waking; that drop helps initiate and deepen sleep.
Regular meal and exercise times reinforce the clock; erratic late-night eating and shift work fight it.
Cortisol up, melatonin down
Your body runs on a daily clock. In the morning cortisol rises to wake you up; in the evening melatonin rises to wind you down. Keeping these on a regular schedule is what makes good sleep feel automatic.
How it works →
The SCN entrains to the light/dark cycle via melanopsin-containing retinal ganglion cells, then conducts peripheral clocks in nearly every organ. The cortisol awakening response and evening dim-light melatonin onset (DLMO) are the two anchor points of the rhythm.
What the studies show →
Chronic circadian disruption (shift work, jet lag, irregular schedules) is associated in large cohort studies with worse metabolic, cardiovascular, and mental-health outcomes — which is why regularity, not just duration, matters.
Regularity beats heroics
A consistent sleep and wake time — even on weekends — keeps your clock aligned so that falling asleep and waking up stop being a fight. Consistency is often a bigger win than any single trick.
Which cue resets your circadian clock most powerfully?
- Meal timing
- Light exposure
- Caffeine
- Room temperature
Show the answer →
B.Light exposure
Light is the dominant zeitgeber. Morning light advances your clock and evening light delays it — which is why light timing is your most powerful sleep tool.