Sleep & the body clock

You spend about a third of your life asleep. That sounds like a lot of lost time, but it is exactly the opposite. During sleep the body recovers, the brain sorts through the day's experiences and stores what you have learned. Anyone who sleeps too little concentrates worse and gets ill more easily.

The inner clock

An inner clock ticks in your head. It runs all by itself in a rhythm of about 24 hours and tells the body when to be awake and when to be tired. Experts call this daily rhythm the circadian rhythm. It steers not only sleep, but also body temperature and when you feel hungry.

Light sets the clock

For the inner clock to run correctly, it needs a signal from outside: light. When it gets dark in the evening, the body releases the hormone melatonin, and you become tired. In the morning bright light slows the melatonin down, and you wake up. Screens shine brightly and carry a high share of blue. This blue light acts on the inner clock like daylight: staring at a phone late in the evening slows the release of melatonin and makes falling asleep harder.

How much sleep somebody needs depends above all on age. In their first year babies reach about 14 to 17 hours a day, school children need roughly 9 to 11 hours, teenagers 8 to 10, and adults 7 to 9 hours. Anyone who stays clearly below that for a long time builds up a sleep debt and notices it in their concentration and their mood.

When the clock gets out of sync

If you fly far to the east or west, your inner clock suddenly no longer matches the local time. You are tired in the middle of the day and wide awake at night. This is called jet lag, and it takes a few days until the clock adjusts to the new time. Flying east is usually the harder direction, because the day gets shorter and you are meant to sleep earlier than you are used to. People who have to work at night experience something similar.