Glossary · Definition
Circadian rhythm
Circadian rhythm is the body's roughly-24-hour internal clock that regulates sleep, hormones, alertness, body temperature, digestion, and more. Light (especially morning sunlight) is the dominant zeitgeber — the signal that sets the clock.
Definition
Circadian rhythm is the body's roughly-24-hour internal clock that regulates sleep, hormones, alertness, body temperature, digestion, and more. Light (especially morning sunlight) is the dominant zeitgeber — the signal that sets the clock.
What it means
Driven by the suprachiasmatic nucleus (SCN) in the hypothalamus, the master clock receives light signals via the eyes (specifically melanopsin-containing retinal ganglion cells). Cortisol peaks 30 min after waking; melatonin starts rising 2-3 hours before sleep onset; core body temperature drops in the late evening. Disruptions (shift work, jet lag, late-night screens) push the entire orchestra out of sync.
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Why it matters
Circadian disruption is associated with increased risk of obesity, type 2 diabetes, cardiovascular disease, and depression. The most-actionable fix is morning sunlight (5-10 min within 30 min of waking) and dimming evening light. These two interventions reliably improve sleep onset by 15-30 min and overall sleep quality.
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Frequently asked questions
Does blue light from screens really disrupt sleep?
Yes, but less than commonly believed for moderate evening screen use. The bigger lever is bright OVERHEAD light close to bed, not phone screens.
Can I shift my circadian rhythm?
About 1-2 hours per day if motivated, via timed light exposure + meal timing. Faster than that produces jet-lag-like symptoms.