rapid-eye-movement sleep: the stage of sleep, formerly called desynchronized sleep, in which most dreaming tends to occur during which electroencephalograms show activity that resembles wakefulness (hence, it is also known as paradoxical sleep) except for inhibition of most skeletal and cranial muscles. This stage has two phases—tonic and phasic—and it is largely during the phasic period that muscle twitches and bursts of rapid eye movements occur. REM sleep accounts for one quarter to one fifth of total sleep time. Compare NREM sleep.