Do you know why you sleep? You may think it's because you're tired. Or perhaps you've been told sleep is good for you. Yet no one knows exactly why or how people sleep. Until the early 1950s, most scientists thought the body and brain rested during sleep. Then a scientist watched people sleep. He noticed that during periods of between 5 and 30 minutes each, sleepers' eyes made rapid movements, as if they were watching something. He called these periods REM sleep, for Rapid Eye Movement. Periods when sleepers' eyes did not move he called non-REM sleep. Whenever the scientist woke people during REM sleep, they were having vivid dreams. Measurements showed that sleepers' pulses went up quickly during REM sleep. Their breathing, heartbeat, and the blood going to the brain also increased. Far from being a time of rest, REM sleep is a time when the body is hard at work. Scientists still don't know the reason for this. Some believe REM sleep makes learning easier. Babies spend a lot of time in REM sleep. Because of this, some scientists believe that REM sleep helps the brain grow. Others think REM sleep is a time for the brain to empty the day's garbage. One thing all the scientists agree on is that sleep is necessary. Without enough sleep, people make mistakes. Most major car accidents happen during the early morning hours, when people are sleepiest. What is the main idea of the article?
REM sleep helps the brain grow. REM sleep has been watched and recorded by scientists. REM sleep is a time to empty the day's garbage. REM sleep is an active time when the body is hard at work.