The causes of side sleeping back pain are ankylosing spondylitis, lumbar disc herniation, local skin infection, lumbar muscle soft tissue damage and so on. 1. Ankylosing spondylitis: Ankylosing spondylitis is a chronic progressive inflammation of the spine, which can cause low back pain at an early stage, which is obvious when sleeping and resting, and is alleviated after activities. 2. Lumbar disc herniation: when sleeping on the side, the spine may be curved in an arch, and if there is a lumbar disc herniation, it may stimulate the nerves and cause low back pain. 3. Local skin infection: If the lumbar local tissue is infected, when sleeping on the side, the infected area is stimulated, which may produce pain. Inflammatory reaction occurs in the infected area, redness, swelling, heat and other manifestations can also occur. 4. Damage to the soft tissue of the lumbar muscle: lie flat to sleep on the side of the process, if the amplitude of movement is too large, it may strain the lumbar muscle, resulting in damage to the soft tissue, causing low back pain. Side sleep low back pain may also have other pathological factors, such as lumbar myofasciitis, etc., it is recommended to go to the hospital in a timely manner, improve the examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment or treatment.