Sleep is usually divided into five phases: stages 1, 2, 3, 4, and REM (rapid eye movement) sleep. These stages progress in a cycle as follows: from stage 1 to REM sleep, then the cycle starts over again with stage 1. Around 50 percent of the total sleep duration occurs in stage 2 sleep, about 20 percent in REM sleep, and the remaining 30 percent in the other stages.
Stage 1 is light sleep, and this is the stage during which the sleeper drifts in and out of sleep and can be easily aroused. Eyes move very slowly and muscle activity slows down. If you get awakened from stage 1 sleep, you may often remember fragmented visual images. Some people may experience sudden muscle contractions called hypnic myoclonia, often preceded by a sensation of being about to fall. These sudden movements are similar to the "jump" we make when we get startled. Upon progressing into stage 2 sleep, eye movements stop and brain waves (fluctuations of electrical activity that can be measured by electrodes) become slower, with occasional bursts of rapid waves called sleep spindles. The hallmark of stage 3 of sleep is extremely slow brain waves called delta waves, interspersed with smaller, faster waves. Stage 4 is composed almost entirely of delta waves. It is very difficult to wake someone during stages 3 and 4, which together are called deep sleep; no eye movement or muscle activity are seen during these two stages. People awakened during deep sleep do not adjust immediately and often feel confused and disoriented for several minutes after they wake up.
Upon switching into REM sleep, respiration becomes more rapid, irregular, and shallow, eye movements become jerky and rapid in various directions, and limb muscles go into temporary paralysis. The heart rate also increases, in parallel with a rise in blood pressure; males develop penile erections. When people awaken during REM sleep, they often describe bizarre dreams.
The first REM sleep period usually occurs about 70 to 90 minutes after falling asleep. A complete sleep cycle takes 90 to 110 minutes on average. The first sleep cycles each night contain relatively short REM periods and long periods of deep sleep. As the night progresses, REM sleep periods increase in length while deep sleep decreases. By morning, people spend nearly all their sleep time in stages 1, 2, and REM.
Stage 1 is light sleep, and this is the stage during which the sleeper drifts in and out of sleep and can be easily aroused. Eyes move very slowly and muscle activity slows down. If you get awakened from stage 1 sleep, you may often remember fragmented visual images. Some people may experience sudden muscle contractions called hypnic myoclonia, often preceded by a sensation of being about to fall. These sudden movements are similar to the "jump" we make when we get startled. Upon progressing into stage 2 sleep, eye movements stop and brain waves (fluctuations of electrical activity that can be measured by electrodes) become slower, with occasional bursts of rapid waves called sleep spindles. The hallmark of stage 3 of sleep is extremely slow brain waves called delta waves, interspersed with smaller, faster waves. Stage 4 is composed almost entirely of delta waves. It is very difficult to wake someone during stages 3 and 4, which together are called deep sleep; no eye movement or muscle activity are seen during these two stages. People awakened during deep sleep do not adjust immediately and often feel confused and disoriented for several minutes after they wake up.
Upon switching into REM sleep, respiration becomes more rapid, irregular, and shallow, eye movements become jerky and rapid in various directions, and limb muscles go into temporary paralysis. The heart rate also increases, in parallel with a rise in blood pressure; males develop penile erections. When people awaken during REM sleep, they often describe bizarre dreams.
The first REM sleep period usually occurs about 70 to 90 minutes after falling asleep. A complete sleep cycle takes 90 to 110 minutes on average. The first sleep cycles each night contain relatively short REM periods and long periods of deep sleep. As the night progresses, REM sleep periods increase in length while deep sleep decreases. By morning, people spend nearly all their sleep time in stages 1, 2, and REM.
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