Sometimes when you lay on your back, it cuts off circulation to the vena cava and can make you feel like you're going to hurl or pass out. You'll wake up if you are in any "danger" to reposition yourself. I prefer to sleep on my back, but I usually have a pillow under half of me so I'm angled a bit and another pillow between my legs.
Usually at the hospital the bed isn't flat so you are propped up.
I also sleep on my side, my belly, I'm pretty much all over. I wake up to pee once a night though.
J- sahm to Z~12.07, A~4.09 and J~ 8.13