Sleeping on stomach vs. on back
My informant notified me that, according to his parents, sleeping face-down is bad. His parents told him that sleeping face-up would prevent his soul from escaping his body while sleeping.
Both of my informant’s parents are Muslim, and he believes that is where the superstition comes from, but cannot recall where his parents learned it. He says they have told him that as a child, so he only sleeps on his back out of habit—not out of fear. He does not believe in the superstition at all, and only thinks of it when thinking of superstitions. When asked why the back is considered safer, he replied “I’m not sure. I think they believe souls escape out of the mouth, so I’m surprised sleeping face up would protect you.”
The orientation of the body may be related the position of Heaven in Islam: if the soul does manage to escape out of the mouth, it would go in the direction of heaven. Perhaps the fear is that the soul could be accessed from below, in which case devils would have a better chance of stealing the soul.
This superstition could also be founded in medical reasoning: sleeping face up protects the natural curvature of the spine. Also, this could assuage parent’s fears of accidental self-smothering, an attempt to prevent children from dying in their sleep.