There is much about sleep that remains mysterious, but experts divide it into two main categories, based on observations of sleeping persons and recordings of the brain’s electrical activity during sleep:
Non-REM sleep– This is divided into four stages, with Stage 1 the lightest and Stage 4 the deepest.
Rapid-eye movement (REM) sleep - During REM sleep, dreaming is common, muscles (other than the eyes) are inactive, and electrical activity in the brain is similar to that of an awake person. The blood pressure and heart and breathing rates may suddenly increase for short periods of time, just as they do during wakefulness
During a typical eight-hour period of sleep, a person drifts from wakefulness to Stage 1 non-REM sleep, through Stages 2, 3 and 4 and finally REM sleep over the first several hours. During the last half of the night, REM sleep and Stage 2 sleep alternate for 90 to 120 minutes each. As we age, brief awakenings increase in frequency, while deeper stages of non-REM sleep decrease.
