I recommend to my post liposuction patients that they wear their compression garments during sleeping and to place a waterproof barrier on their mattress for the first night in case their is drainage, but I don't tell them they can't sleep in a certain position, unless it was on the face and neck, then it is best to avoid sleeping on your stomach or side for the first few nights. If the abdomen is operated on, and you sleep on your tummy there might be some more swelling and bruising and the same would hold for having liposuction on the back and sleeping on your back, but if it's not uncomfortable, I would not restrict my patients from doing so. It is important that you ask your surgeon why you were told this and to ask their permission because there might have been a unique circumstance for you.