In a normal adult, after falling asleep there is a period of about 20 to 30 minutes before entering a long period of slow wave sleep. It is this sleep which is refreshing and does not contain dreams. Subsequent to this there are alternating periods of slow wave and REM sleep, the latter of which is non-refreshing and involves dreams.
During the night the duration of slow wave episodes decreases and the duration of REM increases.
Earlier into sleep it is harder to wake up; it takes a long while to awaken from slow wave sleep.
A sleep deprived individual will only take a matter of minutes to enter slow wave sleep on falling asleep, but even this is a finite amount, which means that anything which interrupts sleep every few minutes will result in loss of the refreshing aspect of sleep.
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