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Scientists have identified distinct stages that your mind and body go through while you sleep. The five stages of sleep are falling asleep, light sleep, two related stages of deep sleep, and rapid eye movement, or REM, while dreaming. If these periods are abnormal, perhaps due to sleep apnea, narcolepsy, or somnambulism, you may be tired, forgetful, or able to remember your dreams more clearly.
Brain activity, taken in the form of waves measured by an electroencephalogram, or EEG scan, identifies the stages of sleep. Other determining factors include breathing rate, heartbeat, muscle activity, how easily you can be awoken, and eye rolling or REM. These are compared to your waking activity when you emit beta waves with high frequency and low amplitude or intensity.
The first two stages of sleep are of a relatively light quality, called transitional periods. During stage I, you relax your muscles, breathe deeply and slowly, slowly roll your eyes, and frequently move your limbs for 5-10 minutes. You might experience hypnagogic phenomena where you jerk as if falling or see morphing pictures. Brain activity has a mixture of beta and theta waves with slightly more intensity. Stage II lasts 10-20 minutes with increasing theta waves that are more synchronous, meaning consistent.
In stages III and IV, lasting 15-30 minutes at a time, almost all your muscles are paralyzed. Your body rejuvenates itself by healing damaged tissue. Your brain predominantly emits delta waves that have low frequency and high amplitude. Stage III has less than 50% delta waves while stage IV has more than 50%, but otherwise they are the same. We feel well rested after experiencing delta sleep, and its difficult to awake us during this period.
Finally, and most interestingly, comes stage V's REM sleep. Although we remain paralyzed, suddenly our brain behaves as if we are awake, with unpredictable dyssynchronous beta waves. Accounting for 20-30% of total sleep time, this stage allows us to dream. Our eyes dart and meander in their sockets as if our dreams were occurring in real life.
Healthy individuals smoothly transition from stage I through stage V at the beginning of the night. Then we return from REM to stage II, III, IV, back to REM, in 90-minute cycles lasting the rest of the night. Scientists still don't quite understand why we dream, but people kept from dreaming develop behavioral and mental problems, so it is clearly a critical behavior.