Q: Why do limbs fall asleep?
A: A limb goes numb when you remain seated or lying in a position that compresses a nerve in the limb or cuts off blood flow to the limb, according to the UC Berkeley Wellness Letter. This can happen when you cross your legs or sleep on an arm for prolonged periods. The tingling "needles and pins" sensation you feel when you move is related to the restoration of sensation. Shaking the limb helps speed recovery.
A related condition called "credit card wallet sciatica" occurs when a man carries a fat wallet in his back pocket and sits on it for more than a few minutes. The pressure of the wallet on the sciatic nerve causes temporary numbness, pain and tingling.