The Stages of Sleep

There are two basic types of sleep:
Non-REM (NREM) sleep – This includes stages 1-4 of sleep.
Stage 1 is very light sleep, and stage 2 is transitional sleep.
Stages 3 and 4 are deep sleep. They occur in the first 3 hours of sleep. Stage 4 is the only restorative sleep phase. It increases energy in the brain cells, releases growth hormone, and builds the immune system.
Dream, REM sleep – mainly starts after 3 hours of sleep. Each 90 minute cycle has a higher percentage of REM sleep. Although the brain only weighs 3 pounds, it uses 25% of our O2. More O2 is used during REM than during waking hours. That is why sleeping 9 or more hours can make you feel mentally drained.
Studies show that learning takes place during sleep. We show more benefit of study/practice in the morning after good sleep.