Slept with head on bicep.arm was not numb but i couldn't lift it more than a few inches when i woke up. it's fine now.what causes this?

Nerve entrapment: when you raise your arm over your head the nerves are trapped under the clavicle.. that's why you can not move your arm..

Arm fell sleep: Your arm fell sleep because of nerve and blood vessel compression. After you re-position and the circulation is restored to the nerves and muscles, their function comes back. You should avoid such positions.