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During sleep your body appears to be inactive. But on the inside, a flurry of activity prepares your mind and body for action. All of this activity occurs during multiple “stages” of sleep that make up one “sleep cycle.” The nature of your sleep is different in each of these stages.
When you first fall asleep, you normally enter non-rapid eye movement sleep. NREM sleep begins with stage N1, a brief stage of light sleep that lasts about 10 minutes or less. A gentle whisper, a soft touch or a slight sound can easily wake you up.
Then you enter stage N2, the second stage of NREM sleep. Your eyes stop moving and your muscles relax. This is followed by the third stage of NREM sleep, stage N3. It is also called “deep sleep” or “slow-wave sleep.” Your brain waves slow down, and it becomes much more difficult to wake you up.
Normally, the final stage of a sleep cycle is stage R sleep, which is rapid eye movement sleep – or REM sleep. During stage R sleep, your brain waves move in a fast pattern. While your eyes remain closed, they flicker rapidly.
Most of your dreams occur during stage R sleep. To protect you from acting out your dreams, your brain paralyzes many of your muscles during this sleep stage. This state of paralysis is called “atonia.”
At the end of a sleep cycle, you start over and re-enter NREM sleep. Each complete sleep cycle lasts about 90 to 110 minutes. Most adults will go through about four to six cycles in a full night of sleep.
Children have much shorter sleep cycles than adults. The sleep cycle of a 1-year-old may last about 45 minutes. By 10 years of age, the length of a child’s sleep cycle is similar to that of an adult.