The clock light sets

About this video

You get sleepy at about the same time every evening, even with no clock in sight. That is your body clock: an inner timer that runs on a cycle of about 24 hours and tells your body when to be sleepy and when to be alert. It also times hunger, body temperature and the release of hormones. These daily cycles are called circadian rhythms.

The master clock is a small group of nerve cells deep in the brain, and it gets signals about light straight from the eyes. Light and dark have the biggest effect on it. In the evening, as it gets dark, the master clock tells the brain to make more melatonin, a hormone that makes you sleepy; in bright light the brain makes little of it.

Left alone, in a room with no daylight, most people's inner clock runs a little longer than a day: about 24 hours and 10 minutes. That is why it needs setting. Every morning, daylight nudges it back in step with the real day, and evening light pushes it the other way, later.

Around puberty the body clock shifts later, by up to about 2 hours. A teenager who isn't sleepy until late is not lazy: their clock is running later than it did a few years before, and with school starting early many teenagers get less sleep than they need.

When you fly across several time zones the wall clock jumps but your body clock doesn't. That mismatch is jet lag. The body clock catches up by about an hour a day after flying east and about an hour and a half a day after flying west, so flying east is usually harder; people who work night shifts face the same mismatch. What helps: daylight in the morning, dim and calm evenings, and waking at about the same time every day. This video goes with the lesson Your Body Clock in Knaph's Health Level 1 course.

Transcript

1. 0:00You get sleepy at about the same time every evening, even with no clock in sight. Something inside you keeps time.
2. 0:08A master clock deep in the brain gets light signals straight from the eyes. In bright light, the brain makes little melatonin. When it gets dark, it makes more, and you feel sleepy.
3. 0:26Without daylight, most people's clock runs about twenty four hours and ten minutes. So every morning, daylight nudges it back. Evening light pushes it later.
4. 0:42At puberty the body clock shifts later, by up to about two hours. A teenager who isn't sleepy until late is not lazy.
5. 0:56Fly across time zones and the wall clock jumps, but your body clock doesn't. It catches up about an hour a day after flying east, an hour and a half after flying west.
6. 1:12Get daylight in the morning. Keep evenings dim. Wake up at the same time every day.
7. 1:22Light keeps your body clock on time.