There are two categories of sleep, non-rapid-eye-movement (NREM) sleep and rapid-eye-movement (REM) sleep.
NREM sleep makes up 75 percent of a normal sleep period. It is divided into four stages: stage one, stage two, stage three, and stage four. Each stage brings on progressively deeper sleep. NREM sleep is an essential time for your body to repair. During NREM sleep, your brain is fairly quiet but your body tends to move more. Growth hormone is secreted during NREM sleep, and this promotes muscle repair and stimulates tissue growth. Interleukins are also released in high amounts. They stimulate your immune system so you stay strong in fighting off infections, and they destroy new cancer cells that develop intermittently in your body. The best stages for your health are the deeper stages three and four, stages during which higher levels of growth hormone and interleukins are released.
REM sleep accounts for about 25 percent of a normal sleep period. It is in this period that you do most of your dreaming. When someone is in REM sleep, you can see that person's eyes darting around beneath the eyelids. During REM sleep, the brain is active but most of the body is paralyzed. This paralysis prevents one from acting out dreams (although dreams do occur to a smaller degree during NREM sleep as well). In REM sleep, your brain consolidates and processes newly learned information. Recent experiences and thoughts are replayed over and over, allowing neural networks to form new paths and make tighter connections. This is an important feature of memory consolidation. If you don't get a good night's sleep, your memory of what you learned that day will probably not be retained for very long. REM is just as important for learning factual information as it is for learning and remembering muscle tasks such as playing a new song on the piano or perfecting a golf swing.
During REM sleep, your brain also solves problems. Many of you have probably had that Aha! experience-the one when you wake up and discover that you suddenly have the solution to a problem that had you stumped the night before. That is the benefit of REM sleep. Your brain truly functions better with a good night's sleep. When you are confronted with a new situation and are not sure what to do about it, "sleep on it."