Most of my lucid dreams take place at night in my house, I don't know what causes this in normal dreams, but normal dreams are made out of your daily thoughts, maybe you are a night person and spend alot of time experiencing the world at night, were as in the day time not much happens for you, therefore you dream about night time, in lucids it could be because you expect it to be night out because it always has been in most dreams, and if you don't like dreams that are so dark, then it could be caused by you fearing it, thus it manifests because you think about it a lot.