REM sleep vs REM

REM sleep and REM both are nouns.

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

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

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

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

REM sleep (noun)	REM (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