I usually have a
whey protein shake mixed with milk. When you go to sleep your body goes without food for a relativley long time (I eat every 3 hours). It's better to take in protein that takes longer to digest, milk & especially casein protein takes a little longer than whey, which is probably best post-workout with water & simple carbs.
Your body needs essentially something to 'work on' while you sleep, most pro-bodybuliders wake themselves after every 3 hours of sleep for a protein boost to stop their muscles from becoming catabolic.