For years, people thought that sleep was unimportant and inactive. Today, we know that sleep is important for good physical and mental （精神的）, the brain is very active.
①By doing this they have learned that there are five stages of sleep. Stage one sleep is very light. During this time, a person wakes up easily. As we move into stage （A）____, our eyes stop moving and our brain waves move more slowly. During stages three and four （often called "deep sleep" ）, it is very hard to wake up a person. During this time, the body repairs itself and makes more cells （细胞）, the body makes a chemical used for growing. In the next stage—REM sleep—we begin to breathe faster and our eyes move around quickly, but （B）我们的身体不动。During this stage, we dream.
The amount of sleep each person needs is different, and is often connected to age. ②（C） In general, babies need fifteen to sixteen hours of sleep a day. Children and teenagers need nine to ten hours, while adults need six to eight. Over time, not having enough sleep—especially deep and REM sleep—can cause one to become sick more easily. Doctors also think that REM sleep is very important for learning, and memory. ③
Scientists are continually learning more （D）____ sleep, but one thing they know for sure is this：for humans, it is as important as food and water.
