late 15c., from Old French nocturnal "nightly, nocturnal," or directly from Late Latin nocturnalis, from Latin nocturnus "belonging to the night," from nox (genitive noctis) "night," cognate with Old English neaht (see night) + -urnus, suffix forming adjectives of time. Nocturnal emission "involuntary ejaculation during sleep" first recorded 1813.
nocturnal noc·tur·nal (nŏk-tûr'nəl)
Of, relating to, or occurring in the night.
Most active at night.