1. People who work or live at night, sleeping during the day. 2. Nonconformists. Popularized by Jean Shepherd, a NY City (and syndicated) radio commentator on the human condition, author, performer, TV documentarian, and film producer including the perennial "A Christmas Story".
Night's a very different world -- familiar things are dark and done with, restrictions on freedom are removed. And that's when the night people flower -- artists, jazz musicians, students and others who prefer that world.