Circadian rhythm: how the 24-hour clock works, what breaks it, and how to reset it

What it means

The circadian rhythm is an internal cycle of about 24 hours generated by a master clock in the suprachiasmatic nucleus of the hypothalamus and by clock genes (CLOCK, BMAL1, PER, CRY) in nearly every cell. The master clock is set each day by light, especially blue-rich morning light; peripheral clocks in liver, muscle and gut are also set by meal timing and activity.

It schedules the cortisol peak on waking, melatonin at dusk, the overnight fall in core temperature, growth hormone in early sleep, and the daily swing in insulin sensitivity, which is highest in the morning and lowest at night.

Why it matters for longevity

Misaligned rhythms — shift work, late nights, irregular meals, evening light — degrade sleep, raise evening cortisol and glucose, promote visceral fat and are linked to cardiovascular disease, diabetes and some cancers. The clock also weakens with age, which is part of why older adults sleep worse. Alignment is the cheapest longevity intervention on this site.

What the evidence says

Shift workers have about 20–40% higher rates of cardiovascular disease and type 2 diabetes in large cohorts, and the WHO classifies night-shift work as a probable carcinogen. In controlled misalignment studies (Scheer 2009), eating and sleeping 12 hours out of phase for ten days raised glucose and blood pressure and reversed the cortisol rhythm in healthy adults.

On the fix: morning bright light advances the clock and improves sleep and mood in trials; consistent wake times improve sleep quality more than total sleep time in some studies; and eating earlier in the day improves glucose handling (early time-restricted eating, Sutton 2018).

How to measure or use it

• Proxies: a wearable's sleep-timing consistency and resting-heart-rate curve, a four-point salivary cortisol profile, and HRV trend. Melatonin onset (DLMO) is the research gold standard and rarely needed.
• Reset: outdoor light within an hour of waking (10–30 minutes), a fixed wake time seven days a week, dim warm light after sunset, last meal three hours before bed, caffeine before noon.
• Judge it on sleep-onset latency, the cortisol awakening response and morning glucose over four weeks.

Related

• Sleep and longevity — the full article
• The sleep protocol — what a plan contains
• Fasting and time-restricted eating — meal timing as a clock signal