Sleeping on your belly is bad for your back. When I sleepover with my friend. She is always flipping over back to side to front to back etc. It doesn't matter how you fall asleep more how you actually sleep after the lights are out (mentally).
For myself I sleep on my back, but when I slept in my old room with my bed up against the wall on the righthand side, I always fell asleep on my side facing away from the wall, however in my new room with my bed not up against a wall I always fall asleep on my back, afaik I'm kinda asleep at the time.