Type of Sleep
Sleep is a cycle of phases, a shift back and forth between rapid eye movement (REM) sleep and non-REM sleep. During REM sleep, memories and thoughts from the day are processed; it is the stage of sleep in which vivid dreams occur. During non-REM sleep, many restorative functions occur. In the deepest phase of non-REM sleep (known as slow-wave sleep), the brain recovers from its daily activities and hormones are released, which help the body rebuild itself from damage done during the day.
After you fall asleep, you cycle through 90-minutes of non-REM sleep followed by REM sleep.