Soreness in the left shoulder while sleeping may be caused by physiological factors, or shoulder muscle strain, frozen shoulder, cervical spondylosis and other factors. 1. Physiological factors: Prolonged heavy physical labor during the day leads to the accumulation of lactic acid metabolites in the left shoulder muscles, and the symptoms of soreness in the left shoulder will appear at night when you sleep. 2. Shoulder muscle strain: prolonged shoulder work or prolonged exposure of the shoulder to cold may lead to shoulder muscle strain, causing aseptic inflammation, and when sleeping at night, pressure or cold, the symptoms of left shoulder soreness will appear, and there may be pain on pressure. 3. Frozen shoulder: it may be related to the degeneration of the soft tissue of the shoulder joint, chronic strain and other factors, when the left shoulder is pressurized while sleeping, or the indoor temperature is low, it can make frozen shoulder aggravate, which will lead to the left shoulder pain and soreness, and when it is serious, it will also have the limitation of activities. 4. Cervical spondylosis: For example, nerve root cervical spondylosis, when sleeping at night, sleeping posture is not appropriate, resulting in nerve root pressure, will cause left shoulder pain, may also be accompanied by upper extremity radiating pain, upper extremity numbness and other symptoms. Sleeping left shoulder soreness, if the symptoms continue to be unrelieved, it is recommended to go to the hospital in a timely manner, under the guidance of the doctor standardized treatment.