lights-out vs nighttime

lights-out and nighttime both are nouns.

Word	Noun	Adjective	Verb	Adverb
lights-out	Yes	No	No	No
nighttime	Yes	No	No	No

lights-out (noun)	nighttime (noun)
(military) signal to turn the lights out	the time after sunset and before sunrise while it is dark outside
a prescribed bedtime