There are three main reasons for pain in the left side of the back when sleeping: 1. When the pillow is too high in the horizontal position, it is easy to cause continuous tension and fatigue in the muscles of the back, such as the rhomboid and scapular muscles, which can lead to strain and pain in the back. 2. When lying on the right side, the upper limbs on the left side are in an internal state, which can lead to continuous tension in the back of the shoulder joint and the muscles of the back for a long time. The muscles of the back are constantly under tension, which can lead to strain and pain. 3, when sleeping without good warmth, the body is in a cold state, which can lead to tighter muscle tissue and reduced blood circulation, which can easily lead to soreness. If the muscles, ligaments and surrounding structures of the back have degeneration and hyperplasia, it is easier for the accumulation of metabolites to occur because of the cold, leading to further aggravation of the left back pain symptoms.