Why do your eyes dart around while you sleep?

Your eyes dart around because, several times a night, you enter a special stage called REM sleep, short for rapid eye movement. In this phase your brain lights up almost as if you were awake, your eyes flick back and forth under closed lids, and you are most likely to have vivid dreams. All mammals and birds have it, and so do some reptiles.

At the same moment, your body does something surprising: it goes almost completely limp. Signals to your muscles are switched off, a state called REM atonia. That's handy, because it stops you acting out your dreams. In people whose atonia fails, a condition called REM behaviour disorder, sleepers really do act out their dreams.

REM was discovered in 1953 by Eugene Aserinsky, a graduate student at the University of Chicago, after long hours watching the eyelids of sleepers, with his professor Nathaniel Kleitman. They linked the eye movements to dreaming, and are now seen as the founders of modern sleep research. Do the eyes follow the dream's images? Nobody knows for sure: people blind from birth, who usually don't dream in pictures, still move their eyes in REM sleep.