Rapid eye movement sleep (REM sleep, REMS) is a unique phase of mammalian sleep characterized by random movement of the eyes, low muscle tone throughout the body, and the propensity of the sleeper to dream vividly. This phase is also known as paradoxical sleep (PS) and sometimes desynchronized sleep because of physiological similarities to waking states, including rapid, low-voltage desynchronized brain waves.

Most of the eye movements in “rapid eye movement” sleep are in fact less rapid than those normally exhibited by walking humans. They are also shorter in duration and more likely to loop back to their starting point. About seven of such loops take place over one minute of REM sleep. Whereas in slow-wave sleep the eyes can drift apart, the eyes of the paradoxical sleeper move in tandem.