Sleeping at night and waking up with numbness in the left hand is mainly due to two reasons. First, due to sleep, long time to keep the left side lying position, it will lead to the left arm blood vessels as well as nerves, such as the musculocutaneous nerve, radial nerve, ulnar nerve, median nerve is obviously squeezed, after a long time will lead to the nerve inside the nerve bundles of relative ischemia, there will be numbness of the hand. Second, due to sleep at night when the pillow is too high, cervical vertebrae physiological curvature straightening, disc herniation, some bony narrowing of the channel will be extruded on the nerve, the nerve root will be squeezed after edema, after a long time due to the pressure is relatively large, will lead to the nerve bundles are relatively in a kind of ischemic state, will also be manifested in the numbness of the hand.