answer:Good question! I think even when we are asleep, we have some subconscious sense of spatial awareness that allows us to stay on our beds! Probably the same reason why if you fall asleep sitting down, you won’t wake up on your side. I noticed that I don’t actually roll side to side on my bed, I just kind of rotate in place. It could also be that beds are simply big enough to accommodate movement while sleeping. Try sleeping on, say, a piano bench and report your findings!