After the first 90 minutes of sleep, you continuously cycle between NREM and REM sleep.
There has always been the question of why do we sleep. One reason why we sleep is because our bodies need time to regenerate the energy that was spent during the time that we were awake. The restorative theory of sleep suggests that sleep promotes physiological processes that restore and rejuvenate the body and mind (Gokcebay & others, 1994). The evidence that supports this theory is the hormones that are secreted while we sleep. The amounts of growth hormones, testosterone, plolactin, and many others increase during the sleep process.
On the other hand, the adaptive theory of sleep states that our sleep patterns are based upon our ability to adapt to the environment. There are many reports of evidence for both theories, but the question is still unanswered. Why exactly do we sleep? .
Beta brain waves are small, fast brain waves. A good example of a beta brain wave is when you are still awake while you are preparing to go to bed. There are also brain waves called alpha brain waves. These waves are a little slower and larger than that of the beta brain waves. These brain waves occur when you start feeling drowsy and your thoughts wonder. This is the time period where you may hear someone call your name, or maybe you feel as though you are falling. These feelings are called hynagogic hallucinations. .
Contrary to what people believe, dreams do not only occur during REM sleep. Dreams occur during all of the stages of NREM sleep as well. Dreams that do occur, however, during REM sleep, are more frequent and tend to last longer than those that occur in the NREM sleep stages. People that wake up suddenly during REM sleep remember up to 90% of the dream. This is why people believe that dreams only occur while in the REM sleep stage.
There are a few sleep disorders that can affect dreams. Sleepwalking, night terrors, and REM sleep behavior disorders are only a few of them.