It is not thought to trigger sleepiness. Sleep consists of two distinct states that alternate in cycles lasting about 90 minutes. These states are termed quiet and active sleep, reflecting differing levels of brain nerve cell activity. With each descending stage, awakening becomes more difficult. After about 90 minutes of quiet sleep, eyes move rapidly behind closed lids, giving rise to the state known as rapid eye movement, or REM sleep.
Most vivid dreams occur in REM sleep.
REM sleep brain activity, which is comparable to that in waking, prompts the designation of REM sleep as active sleep. Quiet sleep commonly is called non-REM or NREM sleep. In REM sleep, skeletal muscles are virtually paralyzed, possibly preventing people from acting out their dreams. With each cycle, NREM sleep becomes progressively lighter, and REM sleep becomes progressively longer, lasting from a few minutes early in sleep to perhaps an hour at the end of the sleep episode. Infants may sleep as many as 16 hours a day.