The night-piece, or 'nocturne' is a classical form. This is something that can either be absorbed into the artist's aesthetic for this project, or ignored. Some typical examples include the 'night music' that appears in several works by Béla Bartók or, before that, Frédéric Chopin's collection of Nocturnes for solo piano. What these works have in common is a poetic evocation of night.
Start by making a list of all the things you associate with night. These do not necessarily have to involve sound. Now start to think about creating mood and atmosphere, about resonance and silence, about presence and sound, about imagination and reality. Fix upon a duration for the piece. Finally, create a piece that evokes night . . .
Night is a particularly good time for listening. It is generally quieter than daytime, and the absence of light means that the ear is heightened. For this project, 'night life' does not mean parties and clubs, but rather the sounds of the night (which might include distant music) that bring it to life.