One possibility is that sleeping on your back is a more vulnerable position. For example animals in a submissive state will expose their bellies. But
in a defensive state they will typically guard themselves by keeping their bellies facing the ground. Predators will often aim for the belly of prey
as well because it's a vulnerable spot. It could be something instinctual. But it's just my guess.
I have trouble sleeping on my back too, so I always sleep face down...to me it just feels more natural and I don't have any problems.



