Rapid Eye Movement Sleep (REM)
There are two main categories of sleep, Non-Rapid Eye Movement Sleep (NREM; which contains stages 1-4; basically everything except REM), and Rapid Eye Movement Sleep (REM). REM sleep is a sleep period during which your brain is very active, and your eyes move in a sharp, back and forth motion as opposed to a slower, more rolling fashion that occurs in other stages of sleep. In addition, during a "normal" night of sleep, people have REM periods every 60-90 minutes throughout the night. These REM periods start off very light and short, but increase in intensity and duration as the night goes on. Your first REM period might be a minute or only a few minutes, but your fourth REM period could last 30 minutes or longer. People often believe mistakenly that humans only dream during REM sleep, although humans also dream during slow wave sleep (stages 3 and 4). However it is true that the majority of our dreaming occurs during REM sleep.