Many people who have frozen shoulder find that their shoulder and arm hurt during the day and even more at night. Some people wake up in pain after falling asleep, and some even have insomnia because of the constant pain! Why is this? According to basic research and clinical observation, the pain of frozen shoulder is caused by the congestion and edema of aseptic inflammation that compresses and strains the peripheral nerves (tendons and ligaments are dense tissues, so even minor swelling can cause strain). On the other hand, inflammatory stimulation, congestion, and blood stagnation cause the accumulation of local inflammatory products (e.g., histamine, 5-hydroxytryptamine, kinins, potassium ions, hydrogen ions, etc.), which themselves have a strong stimulating effect on nociceptive nerves and cause pain. In addition, these inflammatory products can act directly on the smooth muscle of small blood vessels, causing small blood vessel dilation and congestion, increasing swelling and bruising, and stimulating the vessel wall, increasing the permeability of the vessel wall, increasing plasma and leukocyte exudation, and causing inflammation to flare up. When resting at night, the skeletal muscles are in a resting state, in which the tension of the smooth muscles of the small vessels is higher, the resistance to blood flow is higher, and the blood flow is less than during the day (so that the blood nutrition in the body is concentrated on supplying the heart, brain, liver, kidneys and other important organs to safeguard their functional activities). When the blood supply to the inflamed area of the diseased shoulder joint is even lower, the metabolites of inflammation cannot be transported and diluted and decomposed rapidly, and the more they are concentrated locally the higher the concentration, and the stronger the stimulation of the nociceptive nerves. At the same time, due to the increased concentration of these metabolites, the heavier the local swelling and stagnation, the stronger the tension and compression, so the pain in the lesion area will increase at night. Reason 2 The environment is less affected at night During the day, the patient’s attention is distracted by work, study, and interpersonal interactions, while the environment is less affected at night, and the patient’s attention is more likely to focus on the pain in the shoulder, thus feeling more pronounced pain at night. Cause 3 Prolonged pressure on the tissues around the shoulder joint During night sleep, the patient’s posture is fixed and the shoulder capsule or other tissues around the shoulder joint may be pressed or stretched for a long time, thus producing pain.