Sleeping on the side with pain in the middle of the chest may be caused by abnormal sleeping posture, bone fracture or costochondritis. 1. Abnormal sleeping position: sleeping on the side for a long time will cause pressure on the soft tissues on one side of the chest, resulting in poor blood circulation and easily causing pain in the middle of the chest. 2. Fracture: When the chest is injured directly or indirectly by violence, it may lead to fracture of the chest bones (e.g. sternum, ribs). Sleeping on the side will aggravate the fracture and cause pain in the middle of the chest when lying on the side. 3. Costochondritis: Sleeping on the side may aggravate soft tissue irritation, causing the patient to experience pain in the middle of the chest in side-lying. Patients who sleep on their side with a lot of pain in the middle of the chest cannot diagnose a specific disease simply by this symptom. It is recommended to go to the hospital in time for further examination and treatment under the guidance of the doctor.