twenty-four hours vs nighttime

twenty-four hours and nighttime both are nouns.

Word	Noun	Adjective	Verb	Adverb
twenty-four hours	Yes	No	No	No
nighttime	Yes	No	No	No

twenty-four hours (noun)	nighttime (noun)
time for Earth to make a complete rotation on its axis	the time after sunset and before sunrise while it is dark outside