The first time someone said "sleep" and "bra" in the same sentance you freaked, right? Well, yeah, welcome to the maybe not always quite so glamorous side of motherhood, but even Angelina probably wore one of these.
Sizing Stuff: runs a little small, and you're sleeping in it, so go for comfort, it will give you support. 30-32 = S, 34=M, 36=L and remember, this is based on your size before pregnancy.