The suprachiasmatic nucleus, clock genes and the 2017 Nobel Prize, light-sensing ganglion cells, melatonin, jet lag, shift work and why some people are larks and others owls.
Sleep & Consciousness
Beginner
Circadian Rhythms: The Body Clock
The Editors
November 29, 2025
3 min read
In plain English
Your body has an internal clock that runs on roughly a 24-hour cycle, controlling when you feel sleepy or alert, your body temperature and hormone release. A small region of the hypothalamus is the master clock, and morning light resets it each day. Jet lag and shift work happen when this clock is out of step with your schedule.
Beginner level: No background needed. Everyday language, everyday examples — the place to start.
Concepts to know first
Go deeper next
Circadian comes from the Latin circa diem, "about a day". Almost every living thing — from bacteria to plants to people — has internal clocks that anticipate the daily cycle of light and dark.
Clocks without cues
In 1729 the French astronomer Jean-Jacques d'Ortous de Mairan noticed that a mimosa plant kept opening and closing its leaves on a daily rhythm even in a dark cupboard. In 1962 the French caver Michel Siffre lived for two months in an underground cave with no clocks or daylight; his sleep-wake cycle continued, drifting slightly from 24 hours. Human internal clocks typically run a little longer than 24 hours and must be reset daily.
The master clock
In the 1970s researchers identified the suprachiasmatic nucleus (SCN) — about 20,000 neurons in the hypothalamus, just above the optic chiasm — as the master clock. Destroying it abolishes rhythms in rodents; transplanting a donor SCN restores rhythms that follow the donor's period. The SCN synchronises clocks in almost every tissue of the body.
Clock genes
In the 1980s and 90s, Jeffrey Hall, Michael Rosbash and Michael Young, working with fruit flies, showed how the clock works at the molecular level: genes such as period and timeless produce proteins that build up during the day, then shut off their own production, in a loop lasting about 24 hours. They shared the 2017 Nobel Prize. Similar loops, with genes like CLOCK and BMAL1, run in human cells.
Light resets the clock
The clock is reset mainly by light, detected by a special class of retinal ganglion cells containing the pigment melanopsin, which are most sensitive to blue light and connect directly to the SCN. The SCN controls the pineal gland's release of melatonin, which rises in the evening and signals biological night. Bright evening light delays the clock; morning light advances it.
Larks, owls and mismatch
People differ in chronotype: "larks" prefer early hours, "owls" late ones; teenagers shift later. Jet lag occurs when the clock lags behind a new time zone (it usually adjusts by about an hour per day). Shift work chronically misaligns clocks and is associated with health problems.
Everyday examples & things to try
Try it
Reset your clock with morning light
If you struggle to fall asleep at night, try getting outdoor daylight soon after waking for a week and dimming screens and lights in the evening. Morning light shifts the clock earlier — one of the most reliable, free sleep interventions.
Real world
Why eastward jet lag is worse
Because most people's internal clock runs slightly longer than 24 hours, it's easier to delay it (flying west, longer day) than to advance it (flying east, shorter day). That's why many travellers find eastward trips harder.
Key takeaways
- The suprachiasmatic nucleus (~20,000 neurons) is the master circadian clock.
- Clock genes form ~24-hour feedback loops (Nobel Prize 2017).
- Light via melanopsin cells resets the clock; melatonin signals night.
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