We sleep in cycles of 90 to 120 minutes throughout the night. in that time, we cycled through the different stages of sleep. There are four to five sleep stages – non-REM and REM sleep.
Sleep is divided into REM sleep (Rapid Eye Movement sleep) and non-REM sleep. The first 3/4 stages of sleep, are non-REM Sleep, and the fifth is REM Sleep and is when we dream.
We cycle through the different stages of sleep, roughly five times a night – sleep cycles.
Each sleep cycle lasts 90 – 120 minutes and each sleep stage in each sleep cycle usually lasts between 5 to 15 minutes.
For more information, see my blog on Sleep Cycles and Sleep Stages