Longevity Medicine
What Is Circadian Rhythm?
Last reviewed: May 2026 · Haute MD Editorial Team
Circadian rhythm is the roughly 24-hour endogenous oscillation that coordinates sleep timing, hormone release, body temperature, metabolism, and immune function across every cell of the body. Driven by the suprachiasmatic nucleus and entrained primarily by light, it is one of the most fundamental regulators of physiology. Chronic circadian misalignment — shift work, jet lag, irregular schedules — is now recognized as an independent risk factor for cancer, metabolic disease, and accelerated aging.
How the circadian system works
The suprachiasmatic nucleus (SCN) in the hypothalamus acts as the master clock, receiving direct light input from retinal melanopsin cells. The SCN coordinates peripheral clocks in nearly every tissue — liver, muscle, fat, gut, immune cells — each with its own intrinsic oscillation but normally synchronized to the central clock. Morning light raises cortisol and suppresses melatonin, evening darkness reverses this. Core body temperature falls ~1°C overnight and rises before waking. Insulin sensitivity, growth hormone, testosterone, leptin, and inflammatory markers all follow circadian patterns. When peripheral clocks desynchronize from the SCN — as happens with late-night eating, irregular sleep, or shift work — metabolic and hormonal coordination breaks down at the cellular level.
Circadian disruption and disease
Long-term shift workers have elevated rates of cardiovascular disease, type 2 diabetes, breast and prostate cancer (the WHO classifies night shift work as a probable carcinogen), depression, and all-cause mortality. Even non-shift-workers with chronic 'social jet lag' — large weekday-to-weekend timing shifts — show worsened insulin sensitivity, elevated inflammation, and weight gain. Mechanisms include misaligned clock-gene expression in metabolic tissues, suppressed nocturnal melatonin (an antioxidant and oncostatic hormone), and disrupted cortisol rhythm. Aging itself weakens circadian amplitude — older adults often experience phase-advanced, lower-amplitude rhythms with earlier awakening.
How to align circadian rhythm
(1) Get bright outdoor light within 30-60 minutes of waking — the single most powerful entrainment signal; (2) Keep evenings dim — reduce overhead light after sunset, minimize blue-light screens within 1-2 hours of bed; (3) Stabilize sleep and wake times within ~30 minutes daily, including weekends; (4) Front-load eating — finish dinner 2-3 hours before bed, ideally compress the eating window to 8-12 hours; (5) Exercise consistently and earlier in the day when possible; (6) For travel, use morning light, melatonin (0.3-0.5 mg), and timed meals to accelerate adaptation; (7) Avoid alcohol and caffeine timing that disrupts the night. Circadian alignment is foundational — many other longevity interventions work better against a properly entrained clock.
Frequently Asked Questions
How long is the circadian period?
The intrinsic period averages slightly longer than 24 hours — typically 24.1-24.3 hours in adults — which is why daily light cues are required to keep it aligned with the 24-hour environmental day. Without light cues, sleep timing drifts later by a few minutes per day.
How quickly can I shift my circadian rhythm?
Roughly 1 hour per day in either direction with deliberate light, meal, and sleep timing. Eastward travel (advance) is harder than westward (delay). Full reorganization of peripheral clocks (liver, muscle) often lags central clock shifts by additional days.
Is melatonin a sleeping pill?
No — melatonin is primarily a circadian signaling hormone, not a sedative. Low-dose (0.3-0.5 mg) timed 4-6 hours before desired sleep time is effective for phase shifting. Higher doses commonly sold (3-10 mg) are supraphysiologic, often counterproductive, and may suppress endogenous melatonin.
What is social jet lag?
The difference between your sleep timing on workdays versus free days. A consistent ≥2-hour shift is associated with worse metabolic health, weight gain, and elevated cardiovascular risk, even when total sleep duration is adequate.
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