bedtime vs lights-out

bedtime and lights-out both are nouns.

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

As nouns, lights-out is a hyponym of bedtime; that is, lights-out is a word with a more specific, narrower meaning than bedtime:

bedtime (noun)	lights-out (noun)
the time you go to bed	(military) signal to turn the lights out
	a prescribed bedtime