State of rapid-eye-movement sleep. The stage of sleep with highest brain activity, characterized by enhanced brain metabolism and vivid hallucinatory imagery or dreaming. There are spontaneous rapid eye movements, resting muscle activity is suppressed, and awakening threshold to nonsignificant stimuli is high. The EEG is a low-voltage, mixed-frequency, non-alpha record. REM sleep is usually 20-25% of total sleep time. It is also called "paradoxical sleep."