It is clear that sleep is necessary to everyone. Most people rarely think about how or why they sleep.However, we know that if we sleep well, we feel rested. If we do not sleep enough, we often feel tired and irritable.It seems there are two purposes of sleep:physical rest our bodies and our minds.Both are important to our health.Each night we havetwo kinds of sleep:active sleep gives our body the rest that it needs, and prepares us for active sleep,in which dreaming appears. And we are beginning to understand the importance of the dream stage in our sleep cycles Let's look at the two different kinds of sleep.They are quite different.In passive sleep,the body is at rest.The heart slows down. We move very little.And the brain becomes inactive. This stage of sleep is known as slow-wave sleep because the brain waves move in a very slow, regular rhythm.
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