Heaven is right. The reason you stop being able to sleep on your back is because the baby can compress the aorta and vena cava and slow the blood flow, making you feel extremely ill. You will know when you get to the point where you shouldn't be sleeping on your back because you really won't be able to. When it gets to that point, try and train yourself to sleep on your left side because on your right can leave the baby resting on your liver, which is really almost as bad.