nighttime vs period of time

nighttime and period of time both are nouns.

Word	Noun	Adjective	Verb	Adverb
nighttime	Yes	No	No	No
period of time	Yes	No	No	No

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

nighttime: the time after sunset and before sunrise while it is dark outside
period of time: an amount of time

Other hypernyms of nighttime include period, time period.

nighttime (noun)	period of time (noun)
the time after sunset and before sunrise while it is dark outside	an amount of time