A day/night cycle isn't important to me, and I don't think it would the game more immersive. Speeding up time just screws with my own sense of time as my characters do things. If anything, it feels less immersive in that regard.
That aside, if I were suggesting a way to do it, I think I'd start with the idea of saying that it gets dark after two short rests and then gets light again after a long rest.
