Sleep can look like doing nothing. It is not. Your brain and body keep working while you sleep. Sleep helps the body restore energy and save energy.

Sleep helps you pay attention and concentrate. A good night's sleep improves learning and problem-solving skills. Sleep helps your brain work well.

REM sleep is important for learning and memory. Sleep comes in two types. Non-REM sleep goes from light to deep. REM sleep is when most dreaming happens.

Not getting enough sleep makes it harder to think clearly, react quickly and learn.

How much sleep a person needs depends on age, lifestyle, health and sleep patterns. It can differ from person to person. Children from 6 to 12 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.

Animals, plants and even tiny microorganisms have a body clock tuned to the 24-hour day. The daily patterns are called circadian rhythms, and the system that keeps time is called a biological clock.

Light and darkness are the most powerful cues that set the body 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.

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 deep sleep, it is harder to wake someone up.

Enough sleep can help you feel happier and in a better mood. It helps you get sick less often, because it helps the body fight off germs. The body makes growth hormone during sleep, which helps bones and muscles grow.

Dolphins sleep with only half of their brain at a time, so they can stay aware of danger. Animals need sleep too. We all have dreams while we sleep: pictures, thoughts or feelings.

Words to know

• circadian: daily, 24-hour
• melatonin: sleepy-time hormone
• concentrate: focus your mind
• microorganisms: tiny living things