Sleep posture rankings:
1. Sleeping on your back, no pillow. Best posture all around, but hyperlordosis will result if you have poor leg/hip placement.
2. Sleeping on your back, with a pillow. Generally ok, but can promote neck pain, headaches, forward head posture and humpback (thoracic kyphosis) if pillow is thick. Pillow should be no more than about an inch thick when compressed. Again, watch for hyperlordosis.
3: Side sleeping, no pillow, with your bottom arm bent and supporting your head with your hand/forarm. No real issues but requires reasonable shoulder/tricep/wrist flexibility to maintain comfortably. Hips should be aligned to keep the spine straight - pull the bottom side in to keep both of your flanks equal length.
4. Side sleeping, no pillow, with your bottom arm extended to full upen your armpit and your head resting on your bicep. No real issues but depending on head placement can reduce circulation to the arm. Again, watch your hip alignment.
5. Stomach sleeping, no pillow, facial bones resting on your hands to keep your nose from getting crushed. Neck is not neutral but could be worse. Requires reasonable upper back and neck flexibility. Hyperlordosis can be an issue if sleeping on a soft surface. Hyperlordosis can be avoided by bringing one knee up to the side at hip height.
6. Side sleeping, with a pillow, arm perpendicular to your chest. Causes slumped shoulders, caved in chest, and frequent shoulder pain. Again, watch your hip alignment.
7. Stomach sleeping, with a pillow. Very poor neck posture while suffocating yourself at the same time. Again, watch for hyperlordosis.