Did you know you go on a journey every night after you close your eyes? Sleep scientist Matt Walker breaks down the difference between REM (Rapid-Eye Movement) and non-REM sleep, what occurs during each stage of sleep -- and why it's important to get enough of both. Sleeping with Science, a TED series, uncovers the facts and secrets behind our nightly slumber. Check out more episodes on TED.com: https://go.ted.com/sleepingwithscience