text: what happens during rem sleep?   what is rem sleep?  usually, rem sleep occurs 90 minutes after sleep onset. the first period of rem typically lasts 10 minutes, with each recurring rem stage lengthening, and the final one may last up to an hour. polysomnograms show brainwave patterns in rem to be similar to that recorded during wakefulness. in people without sleep disorders, heart rate and respiration speed up and become erratic during rem sleep. during this stage the eyes move rapidly in different directions.  intense dreaming occurs during rem sleep as a result of heightened brain activity, but paralysis occurs simultaneously in the major voluntary muscle groups. rem is a mixture of encephalic (brain) states of excitement and muscular immobility. for this reason, it is sometimes called paradoxical sleep.  the percentage of rem sleep is highest during infancy and early childhood. during adolescence and young adulthood, the percentage of rem sleep declines. infants can spend up to 50% of their sleep in the rem stage of sleep, whereas adults spend only about 20% in rem.