What is How to Measure Sleep?

As we sleep, our brains pass through a series of stages, like shifting between gears in a car. The most famous is REM sleep, characterized by fluttering eye movements. But other categories include light sleep (stage S1), stable sleep (S2), slow-wave sleep (SS), and even being awake (W), each identified by a signature from polysomnography (PSG), in which electrodes record from the scalp, eyelids, and heart.