Definition: This is one of the two basic states of sleep and is notable for a presence of rapid eye movement (REM). It is a deep stage of sleep with intense brain activity in the forebrain and midbrain. It is characterized by dreaming and the absence of motor function with the exception of the eye muscles and the diaphragm. It occurs cyclically several times during sleep, but it comprises the smallest portion of the sleep cycle.
Also Known As: Paradoxical sleep, Dreaming sleep
If you watch closely, you can see his eyelids fluttering during REM sleep.