mornight vs period of time

mornight and period of time both are nouns.

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

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

Other hypernyms of mornight include period, time period.

mornight (noun)	period of time (noun)
both morning and night, used when greeting someone	an amount of time