Non-REM sleep (NonREM sleep or just NREM) which is another name for Non-rapid eye movement sleep refers to type of sleep that is divided into four (4) stages.
Usually about 75% of the night is spent in NREM Sleep. Non-REM sleep lasts about four to six hours.
The four (4) stages of Non-rapid eye movement sleep are:
1) In stage one of the sleep cycle brain waves are referred to as theta waves. They consist of a 4-7 cycle per second rhythm.
2) In stage 2 of sleep, the brain generates sleep spindles. Spindles are a 12-14 rhythm that lasts a half of a second. Sleep talking usually occurs during stages 1 and 2 of sleep. Sleep talking is mumbled and usually not understandable.
3) Delta waves are produced from the brain in the third stage of sleep. These brain waves become slower when the sleep cycle begins. During this cycle your heart rate, blood pressure, and arousal decline.
4) Stage four is very similar to stage 3 because Delta waves continue in the brain. During this stage of sleep most dreams and nightmares occur.