Regularly occurring periods of eye motility, and concomitant phenomena, during sleep.

Aserinsky, one of Kleitman's graduate students, decided to hook sleepers up to an early version of an electroencephalogram machine, which scribbled across ¹⁄₂ mile (800 m) of paper each night. In the process, Aserinsky noticed that sleepers went through periods when their eyes darted wildly back and forth several times each night. Kleitman and Aserinsky introduced the world to "rapid-eye movement," or REM sleep, and demonstrated that REM sleep was correlated with dreaming and brain activity.