How do circadian rhythms work?
Your circadian rhythm is a roughly 24-hour internal clock driven by the suprachiasmatic nucleus in the brain. It synchronizes hormones, body temperature, and alertness to environmental light and time cues.
The master clock
The suprachiasmatic nucleus (SCN), a tiny cluster of neurons above the optic chiasm, receives direct input from light-detecting retinal cells. It uses that input to align internal cycles to the external day.
Downstream effects
The SCN drives melatonin release (sleep signal), cortisol peaks (wake signal), body temperature curves, digestion timing, and dozens of other rhythms. Disrupting the SCN's light input (shift work, jet lag) disrupts all of these.
Keeping it stable
Consistent wake time + morning bright light + dim evening lights is the strongest single intervention for circadian health.
Sources
- Czeisler et al., Sleep, 2004. Sleep deprivation and circadian disruption
- Matthew Walker, Why We Sleep (Scribner, 2017). Why We Sleep: Unlocking the Power of Sleep and Dreams
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