re: Pain in elbow where forearm meets at the top and numb hands and arms when I wake up.

Make sure you read TigerAddict's comment and link. If the numbness is in your pinky and ring finger, it's probably ulnar compression. Sleeping with your hands behind your head can increase compression on that nerve by near 50% with no other complications. If you go to the doctor with that, they'll probably proscribe something like gabapentin, but that doesn't address what causes the compression.

The ulnar nerve runs through the pinkie side of your elbow. If the numbness is in your other fingers, it's probably the nerve on the other side of the elbow. This isn't a big deal.

Fix your sleep style, and potentially mattress. Behind the neck dumbbell presses are not great for the nerves in your elbow (which lead to your hand) either.