rapid eye movement sleep vs rapid eye movement

rapid eye movement sleep and rapid eye movement both are nouns.

Word	Noun	Adjective	Verb	Adverb
rapid eye movement sleep	Yes	No	No	No
rapid eye movement	Yes	No	No	No

As nouns, rapid eye movement sleep and rapid eye movement are synonyms defined as:

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

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

rapid eye movement sleep (noun)	rapid eye movement (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