Non-rapid eye movement sleep: NREM (non-rapid eye movement) sleep is dreamless sleep. During NREM, the brain waves on the electroencephalographic (EEG) recording are typically slow and of high voltage, the breathing and heart rate are slow and regular, the blood pressure is low, and the sleeper is relatively still. NREM sleep is divided into 4 stages of increasing depth leading to REM sleep.
REM sleep is when dreams occur. We have 3 to 5 REM periods per night. They occur at intervals of 1-2 hours apart and are quite variable in length, ranging from 5 minutes to over an hour. REM sleep is characterized by rapid, low-voltage brain waves, irregular breathing and heart rate and involuntary muscle jerks.
About 80% of sleep is NREM sleep. If you sleep 7-8 hours a night, all but maybe an hour and a half is spent in dreamless NREM sleep.