Shoulder pain when sleeping on the side may be caused by cervical spondylosis, myofasciitis or frozen shoulder. 1. Cervical spondylosis: cervical spondylosis is due to long-term strain on the cervical spine, osteophytes, ligament thickening and other factors, stimulate the sympathetic nerves, compression of the nerve roots, and ultimately caused by degenerative cervical spine lesions, the patient is usually manifested in the back of the neck pain, stiffness, numbness of the limbs, if you sleep on your side at night, so that the neck bends to compression of the nerves, which would cause the shoulder pain. 2. Myofasciitis: patients may also suffer from myofasciitis due to trauma, muscle strain, cold and other factors. If the starting point of myofasciitis is in the neck and shoulder position, when sleeping on the side at night, it will stimulate the starting point and cause shoulder pain. 3. Frozen shoulder: the etiology and pathogenesis of frozen shoulder is still unclear, mainly in the soft tissues around the shoulder joint for unknown reasons, self-limiting aseptic inflammation, the patient usually manifests as pain around the shoulder joint and limited activities, sleeping on the side of the shoulder joint at night, pressure, which will aggravate the symptoms of shoulder pain. As the causes of shoulder pain when sleeping on the side of more reasons, may also be angina, pleurisy, herpes zoster and other diseases, if the pain symptoms are particularly pronounced or persistent, it is recommended to go to the hospital in a timely manner for examination and diagnosis, to clarify the cause of the disease to take targeted therapeutic measures, to avoid delaying the condition caused by the aggravation of the symptoms.