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Adjective: nocturnal  nók'tur-n(u)l
  1. (biology) belonging to or active during the night
    "nocturnal animals are active at night"; "nocturnal plants have flowers that open at night and close by day"
     
  2. Of or relating to or occurring in the night
    "nocturnal darkness"

Antonym: diurnal

Encyclopedia: Nocturnal