Sleep is divided into two major states: non-REM and REM. Non-REM sleep has three stages: stage one, which is a dozing state with a fast gamma rhythm; stage two, which has sleep spindles and K complexes and is a conversation between the thalamus and the cortex; and stage three, slow wave sleep, which is the most impressively different and cleans our brain. REM sleep is the most popular because that's where we have the most active dreams. A perfect night's sleep is seven and a half to eight hours, with the first REM sleep period coming about 95 minutes after we fall asleep and lasting about 10-15 minutes. We cycle through the stages of sleep every 90 minutes or so.