Sleeping on your stomach puts tremendous stress on your lower back as well as your neck. Lying on your stomach increases the curvature in your lower back and as you sleep with your head turned it puts stress on your neck. The best sleep position is on your back with a cervical(neck pillow) under your head. Do not sleep with your head propped up. The next best position is on your side with a cervical pillow to keep your neck in line with your spine.