In 1957, Eugene Aserinsky, a graduate student in physiology at the University of Chicago, discovered REM sleep while monitoring the brain waves of his nine-year-old son during nap time. Aserinsky found that when his son entered REM sleep, his eyes moved rapidly from side to side beneath his eyelids. Since then, researchers have identified five key characteristics of REM sleep: Eyes move rapidly from side to side. Heart rate and blood pressure increase. Muscles become temporarily paralyzed. Brain waves show a mix of fast and slow activity. People typically experience intense dreaming during REM sleep.