How does your body know what time it is?
A tiny master clock in your brain keeps a rhythm of roughly 24 hours. Every morning, daylight hitting your eyes resets it to the real time. The clock then tells the rest of your body when to be alert, hungry, warm or sleepy, partly by switching the sleep hormone melatonin on and off.
Daylight sets the clock
Special cells in your eyes don’t help you see, they just measure how bright it is. Morning light sends a signal straight to the clock in your brain and nudges it into step with the day.
Share this stepA tiny clock deep in the brain
The master clock is a cluster of about 20,000 nerve cells just above where the nerves from the eyes cross. Its cells run on a cycle of roughly 24 hours, even in total darkness, and it keeps the whole body on schedule.
Share this stepAt night it signals sleep
When it gets dark, the clock tells the brain to release melatonin, the hormone that makes you sleepy. Body temperature drops a little, digestion slows and you start winding down. In the morning it switches the other way.
Share this stepEvery organ has its own clock
Your liver, gut, muscles and skin also have small clocks in their cells. The master clock keeps them in sync, which is why meals and sleep at odd hours can make you feel off.
Share this stepLight sets the clock, the clock runs the body.
Morning lightmaster clockyour daily rhythm
Jet lag happens when your body clock is still on the old time zone. It usually shifts about one hour per day, and morning daylight in the new place helps it catch up faster.
Common follow-up questions
Why do screens make it hard to fall asleep?
Bright light in the evening, especially bluish light, tells your clock it’s still daytime and delays melatonin. Dimming screens and lights in the evening helps.
Are some people really night owls?
Yes. Genes affect whether your clock runs a little early or late. Teenagers also naturally shift later, which is why early mornings are hard for them.
How do I get over jet lag faster?
Get daylight in the morning at your destination, avoid bright light in the evening, and try to eat and sleep on local time from the first day.
Does the clock work without daylight?
Yes, it keeps ticking, but it drifts a little each day without light to reset it. Studies of people living in caves without clocks showed their days slowly stretching.
