There are two types of sleep, REM and NREM (non-REM). REM, which is also known as paradoxical sleep, stands for Rapid Eye Movement and occurs in cycles every 60-90 minutes througout your sleep period. This means that every 60-90 minutes you enter a REM stage during which you have rapid eye movements and your muscles become almost paralyzed (this is why it’s called paradoxical sleep – the rest of your body is active but your muscles are inactive). The majority of dreams occur in REM sleep, but not all of them.