day vs nighttime

day and nighttime both are nouns.

Word	Noun	Adjective	Verb	Adverb
day	Yes	No	No	No
nighttime	Yes	No	No	No

day (noun)	nighttime (noun)
a period of opportunity	the time after sunset and before sunrise while it is dark outside
time for Earth to make a complete rotation on its axis
a day assigned to a particular purpose or observance
the time after sunrise and before sunset while it is light outside
a point or period in time
the recurring hours when you are not sleeping (especially those when you are working)
an era of existence or influence
the time for one complete rotation of the earth relative to a particular star, about 4 minutes shorter than a mean solar day
the period of time taken by a particular planet (e.g. Mars) to make a complete rotation on its axis