REM vs REM sleep

REM and REM sleep both are nouns.

Word	Noun	Adjective	Verb	Adverb
REM	Yes	No	No	No
REM sleep	Yes	No	No	No

As nouns, REM and REM sleep are synonyms defined as:

REM and REM sleep: a recurring sleep state during which dreaming occurs; a state of rapidly shifting eye movements during sleep

Other synonyms of REM include paradoxical sleep, rapid eye movement, rapid eye movement sleep.

REM (noun)	REM sleep (noun)
a recurring sleep state during which dreaming occurs; a state of rapidly shifting eye movements during sleep	a recurring sleep state during which dreaming occurs; a state of rapidly shifting eye movements during sleep