sleep late vs slumber

sleep late is not a noun while slumber is a noun.

Word	Noun	Adjective	Verb	Adverb
sleep late	No	No	Yes	No
slumber	Yes	No	Yes	No

As verbs, slumber is a hypernym of sleep late; that is, slumber is a word with a broader meaning than sleep late:

Other hypernyms of sleep late include catch some Z's, kip, log Z's, sleep.

sleep late (noun)	slumber (noun)
	a dormant or quiescent state
	a natural and periodic state of rest during which consciousness of the world is suspended

sleep late (verb)	slumber (verb)
sleep later than usual or customary	be asleep