wedding night vs nighttime

wedding night and nighttime both are nouns.

Word	Noun	Adjective	Verb	Adverb
wedding night	Yes	No	No	No
nighttime	Yes	No	No	No

As nouns, nighttime is a hypernym of wedding night; that is, nighttime is a word with a broader meaning than wedding night:

wedding night: the night after the wedding when bride and groom sleep together
nighttime: the time after sunset and before sunrise while it is dark outside

Other hypernyms of wedding night include dark, night.

wedding night (noun)	nighttime (noun)
the night after the wedding when bride and groom sleep together	the time after sunset and before sunrise while it is dark outside