A night owl was a term for somebody who stayed up late at night.
In 2370, when Arjin expressed his misgivings about introducing himself to Jadzia Dax late at night, Julian Bashir assured him she was a night owl. (DS9: "Playing God")
In 2374, The Doctor, upset at the prospect of sharing USS Voyager's sickbay with sleeping crew members, explained that he was a night owl and their presence would be intrusive. (VOY: "Demon")