Sleeping with numbness in the left arm, the following causes are common: 1, improper sleep posture: during sleep, the left upper limb is continuously lying to the left side, pressing the left upper limb under the body, resulting in impaired blood circulation in the left upper limb, and nerve ischemia and hypoxia, and numbness in the left upper limb will appear after waking up. The symptoms of left upper limb numbness can be gradually relieved by muscle massage, hot compresses and moving the limbs. 2. Cervical spondylosis attack: If the patient has a cervical spondylosis attack after sleep, it can also cause symptoms of left upper limb numbness, accompanied by neck pain and limited activity, and in severe cases, it can also cause a feeling of weakness in the left upper limb, and the diagnosis can be confirmed by cervical spine nuclear magnetic examination. Patients experiencing numbness in the left arm in sleep must be diagnosed and treated promptly to avoid continued aggravation of symptoms and adverse consequences.