This is a terrible posture to assume. Sleeping on your stomach will damage the lumbar spine, cervicle spine and the Jaw.  Over the years I would guess that I have had thousands of new patients come to the clinic from specifically injuring themselves from sleeping on the stomach.

Usually people do this because they feel like they can not go to sleep unless they are on their stomach.  Try to buy a body pillow and huge it.  In most cases the feeling of needing to sleep on stomach is a security issue.  Example only when an animal is totally trusting or secure does it lay on its back.

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