There are several causes of arm pain after sleeping: First, myofasciitis. Myofasciitis often starts with pain on the inside of the scapula, which can radiate to the ipsilateral shoulder and upper limb, and when the arm is pressured at night, the arm pain is very obvious. Second, shoulder joint disease, when the shoulder joint has rotator cuff tears or frozen shoulder, there can be pressure on the arm when sleeping and the symptoms of arm pain. Third, cervical spondylosis, cervical spondylosis can compress the nerve root and cause pain in the arm. When sleeping at night, due to the incorrect posture of the neck, the nerve is severely compressed, and arm pain can occur.