Soreness in the arm while sleeping may be caused by improper sleeping position, overwork or due to frozen shoulder or rotator cuff injury. 1. When sleeping on one side for a long time, one side of the armpit is under pressure for a long time, which makes the brachial plexus nerves and blood vessels in the armpit under pressure, resulting in soreness and pain symptoms. 2. Sore arms during sleep may also be induced by overwork during the day or regular lifting of heavy objects on the arms. 3. Sore arms may be caused by frozen shoulder. Frozen shoulder has a wide range of pain, usually including pain on the outer side of the upper arm, armpit, scapular region before and after the shoulder joint and neck and shoulder, and its pain will be aggravated at night. 4. In middle-aged and old-aged patients, due to the degeneration of rotator cuff and tendon, the brittleness of rotator cuff is increased, thus localized pain and lifting weakness will occur. The pain is more pronounced at night at the shoulder joint, acromion, and anterior and posterior aspects of the shoulder joint. When arm pain occurs and is not relieved for a long time, you should consult a doctor for examination and treatment.