sit up vs sit-up

sit up is not a noun while sit-up is a noun.

Word	Noun	Adjective	Verb	Adverb
sit up	No	No	Yes	No
sit-up	Yes	No	No	No

sit up (noun)	sit-up (noun)
	a stomach exercise in which a person sits up from a supine position without using the arms for leverage

sit up (verb)	sit-up (verb)
not go to bed
change to an upright sitting position