Let us know more about arm falling asleep. Arm Falls Asleep While Sleeping: Causes. These sensations occur when your nerves are under pressure. While you keep your arm under your head, you experience this sensation, because your sleeping position exerted pressure on the nerves. Another cause of arm falling asleep is hanging your hand over the back of the chair while you are sitting on it. Other causes of arm falling asleep include anxiety, depression, anemia, B12 or folate deficiency, hyperventilation, stroke and multiple sclerosis.