Home › Rest & recovery › Circadian rhythm

What it is and how it works

Almost every living thing, from bacteria to humans, has internal clocks that anticipate the cycle of day and night. In humans, a small area of the brain called the suprachiasmatic nucleus, sitting just above where the optic nerves cross, acts as the master clock. Most organs, including the liver, gut and muscles, have their own local clocks that follow its lead.

Left completely alone, without daylight or clocks, the human internal cycle runs at slightly more than 24 hours. Each day it needs to be nudged back into step. Signals that do this are called zeitgebers, German for "time givers".

Light is the strongest of them. Special cells in the eye, separate from those used for vision, detect light, especially blue-rich daylight, and report it to the master clock. Morning light tends to move the clock earlier; bright light late in the evening tends to push it later. Meals, physical activity, temperature and social routines are weaker but real signals; see meal timing.

What follows the clock

• Melatonin, a hormone released by the brain in the evening as darkness falls, signals biological night.
• Cortisol rises in the early morning, part of the body's preparation for the day.
• Body temperature is lowest in the early hours and highest in the late afternoon.
• Alertness typically dips after lunch and again late at night.

Chronotypes

Not everyone's clock runs at the same time. Some people naturally wake early and feel best in the morning ("larks"); others are more alert in the evening ("owls"). These tendencies, called chronotypes, are partly genetic and change with age: teenagers tend to run late, while many older adults shift earlier.

When the clock is out of step

• Jet lag happens when you cross time zones faster than the clock can adapt, often about one day per hour of time difference.
• Social jet lag describes the gap between weekday and weekend schedules.
• Shift work asks people to be awake when the clock says night. It is a common challenge, and workplaces and doctors can offer guidance for people affected.

In everyday life

• Get outside in the morning, even on cloudy days; outdoor light is far brighter than indoor light. It is also when the skin can make vitamin D in summer.
• Keep wake-up times steady, weekends included; see sleep hygiene.
• Dim the lights in the evening and keep the bedroom dark.
• Mind the timing of caffeine, which can delay sleepiness.
• Eat at regular times, with the last main meal not too close to bed.

Common confusions

"Circadian rhythm and sleep are the same thing." The clock influences far more than sleep, and sleep is also driven by sleep pressure.

"Being a night owl is laziness." Chronotype is partly biological. Schedules can be shifted gradually, but not everyone is built for very early mornings.

"Blue light from phones is the main problem." The light from screens is modest compared with daylight. The stimulation of what we are doing on them matters too.

People with persistent problems sleeping at the times they need to, or who struggle with shift work, can ask a doctor about sleep and circadian rhythm disorders.

Added Sep 18, 2026 · Filed under Rest & recovery