Sleep gives your body and mind a rest, so you feel fresh when you wake up. It helps children grow. It also helps your brain work well.

You may think your brain turns off at night. It does not. Your brain and body keep working while you sleep.

Animals, plants and even tiny microorganisms have a body clock tuned to the 24-hour day. Earth has a 24-hour cycle of day and night.

The daily patterns that follow the 24-hour day are called circadian rhythms. The system that keeps time is called a biological clock.

When the sun goes down, your brain starts making a sleepy-time hormone called melatonin. Light from phones, tablets and other screens at night can confuse the body's clock. It helps to turn screens off 30 minutes before bed.

Sleep comes in two types. Non-REM sleep goes from light to deep. REM sleep is when most dreaming happens. By the time children start school, one full sleep cycle lasts about 90 minutes, like an adult's. A night holds four to five cycles.

In deeper sleep, the heart and breathing slow down and body temperature drops. It is harder to wake someone up. In REM sleep the eyes move quickly under the eyelids, and the most vivid dreams happen.

How much sleep do you need? Children from 6 to 12 years old need 9 to 12 hours a day. Teenagers from 13 to 18 need 8 to 10 hours a night. Most healthy adults need 7 to 9 hours.

Not getting enough sleep makes it harder to think clearly, react quickly and learn. Enough sleep can help you feel happier and in a better mood.

Dolphins sleep with only half of their brain at a time, so they can stay aware of danger. Animals need to sleep too.

Words to know

• circadian: daily, 24-hour
• melatonin: sleepy-time hormone
• hormone: body messenger
• cycle: a repeating pattern

The short version

People sleep so the body and mind can rest and feel fresh. Sleep helps children grow and helps the brain work well. Sleep comes in non-REM and REM types, and most dreaming happens in REM sleep. A body clock tuned to the 24-hour day tells us when to sleep.

Quick facts

• Children from 6 to 12 years old need 9 to 12 hours of sleep a day.
• Teenagers from 13 to 18 need 8 to 10 hours of sleep a night.
• One full sleep cycle lasts about 90 minutes, and a night holds four to five cycles.
• Dolphins sleep with only half of their brain at a time.
• Turn screens off 30 minutes before bed.