Stage 3 & 4
Stage 3 - This stage represents the threshold to deep sleep. It usually occurs between thirty to forty-five minutes after starting to sleep. In stage 3, extremely slow brain waves called delta waves begin to appear. They are interspersed with smaller, faster waves. In stage 3 the sleeper is much more difficult to waken.
Stage 4 - This stage represents "deep sleep". Delta waves are the primary waves recorded from the brain. It is during deep sleep that the body gets its most rest and does its best repair work. A sleeper in deep sleep will be very difficult to awake.
Stage 3 and 4 are distinguished from each other only by the percentage of delta activity. Together they represent up to 25% of total sleep time. During stages 3 and 4 all eye and muscle movement ceases. The first deep sleep period of the night is the longest and the deepest. After the deep sleep period is over the sleeper reverses the pattern, moving back up to non-REM stage 2 then entering REM sleep.
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