NonREM sleep is also termed quiet sleep. With each descending stage, awakening becomes more difficult. It is not known what governs NonREM sleep in the brain. A balance between certain hormones, particularly growth and stress hormones, may be important for deep sleep. REM sleep is termed active sleep and is believed by some experts to be regulated by the circadian clock in the hypothalamus.
Most vivid dreams occur in REM sleep.
REM-sleep brain activity is comparable to that in waking, but the muscles are virtually paralyzed, possibly preventing people from acting out their dreams. In fact, except for vital organs like lungs and heart, the only muscles not paralyzed during REM are the eye muscles. When people are sleep-deprived, their brains must work harder than when they are well rested. With each cycle, NonREM 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.