夜：when it is dark. 晩：when at night and people are awake. At 8. p.m when the family has dinner, it is 晩, also 夜. At 3 A.M when people are asleep, it is 夜 but not 晩.

I think they are almost same. We often say "今夜" and "今晩", and both mean "tonight". We also say 夜ごはん( pronounced よるごはん) and 晩ごはん. I don't know the differences between them. I guess that 晩 is older Japanese than 夜, and it sounds more formal.