Dean- He sleeps on his side, while you sleep on your stomach. His arm is always underneath your head, mainly because you use it as a pillow, and his other arm is laying on top of your back.
Sam- You both sleep on your sides, while his arm is wrapped around your waist holding your hand and his other arm underneath your head.
Castiel- He doesn't sleep but he does cuddle with you until you fall asleep and is always there when you wake up in the morning. He lays on his back with your head on his chest and he wraps his arms around you so he can hold you as you fall asleep.