What is the antonym of diurnal?

An antonym to diurnal is nocturnal when diurnal is defined;

of or belonging to or active during the day; "diurnal animals are active during the day"; "diurnal flowers are open during the day and closed at night"

Another antonym to diurnal is aperiodic (vs. periodic) when diurnal is defined;

having a daily cycle or occurring every day; "diurnal rotation of the heavens"

~gpack