Sleep has stages. Stage 1 is the short dozing-off stage. Stage 3 is deep sleep. REM sleep is when most dreaming happens.
Sleep also comes in two types: non-REM sleep, which goes from light to deep, and REM sleep. REM stands for rapid eye movement. The most vivid dreams happen in REM sleep, when the eyes move quickly under the eyelids.
In deeper sleep the heart and breathing slow down and body temperature drops. It is harder to wake someone up. 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.
You may think your brain turns off during sleep. During REM sleep brain activity rises to levels like being awake. Breathing and heartbeat speed up.
During REM sleep most muscles are paralyzed. That keeps us from acting out our vivid dreams.
REM sleep is important for learning and memory. Sleep helps you pay attention and concentrate, and a good night's sleep improves problem-solving skills.
During sleep the body releases hormones, including growth hormone, that help it grow and repair damage. Sleep also helps the body restore energy and save energy.
How much sleep a person needs depends on age, lifestyle, health and sleep patterns. Children from 6 to 12 need 9 to 12 hours a day, and teenagers from 13 to 18 need 8 to 10 hours a night.
The human master clock is a group of nerve cells in the brain called the suprachiasmatic nucleus, or SCN. It runs on a cycle slightly longer than 24 hours. Light and darkness are the most powerful cues that set the body clock.
Blue light from phones, tablets and computers can hold back melatonin, the hormone that makes us sleepy. Turning screens off 30 minutes before bed helps.
Words to know
- paralyzed: cannot move
- hormones: body messengers
- nucleus: cell group
- melatonin: sleepy-time hormone
