I haven't seen precise comparisons of # of hours worn before bed and their effects. My advice would be to experiment -- I'm guessing that more hours worn before bed improves performance up to a point, but that's just a guess. There's certainly no point in putting them on before it gets dark outside, unless you're trying to wake up at 3am.

No reason to sleep with them on -- your eyes are closed and it's presumably dark in your bedroom at night.