Shoulder pain when lying down may be caused by the following reasons. 1, shoulder joint trauma, local strain in the process of lying down, will trigger the emergence of pain. 2, cervical spondylosis, triggered by the occurrence of shoulder pain when lying down, because the nerve roots of the cervical spine is stimulated by compression, triggering the emergence of pain around the shoulder joint. 3, frozen shoulder, triggered by the emergence of pain when lying down. If the pain is serious, you can also take oral non-steroidal anti-inflammatory and analgesic drugs to relieve the pain.