Stages of Sleep

There are two main kinds of sleep: nonrapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. NREM sleep is divided into four stages, based on the amount and frequency of different types of brain waves (electrical activity of the brain). Stage 1 occurs as people become drowsy and fall into light sleep. Stage 2, where most sleep time is spent, is a continuation of light sleep. Stages 3 and 4 are the deepest levels of sleep and are necessary for feeling rested.

REM sleep is a very deep but active stage of sleep in which people dream, their heart rate and breathing speed up and slow down, and their eyes move rapidly. In younger people, REM sleep usually follows stage 4 sleep, then alternates with NREM sleep through the night. Many older people reach the deepest sleep (stage 4) slowly, if at all, and spend less time in REM sleep

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