Shoulder pain after waking up is often caused by the following reasons: 1. It may be caused by cold in the shoulder joint. If the shoulder is not covered by clothes and blankets during sleep, it may lead to aseptic inflammation inside the shoulder joint, which may cause shoulder pain after waking up. 2. It may be caused by poor posture. If the patient has a bad posture while sleeping, it will cause certain muscles in the shoulder to be under tension for a long time. Over time, spasms around the muscles may occur, and the patient may feel sore after waking up. 3. It may be caused by overexertion. If the patient does very intense activities before sleeping, resulting in very fatigued shoulder muscles, there is also a possibility of soreness after waking up. 4. It may also be caused by excessive pressure for a long time. Some patients are used to sleeping on one side or in a prone position, which may cause the shoulder joint to be squeezed for a long time and cause the patient to wake up with shoulder pain.