Have you checked for arthritis and spondylitis tests? I think you should check with your doctor, huge chances of spondylitis. My sister also had similar symptoms was tested positive for spondylitis.

With your arms up you are tractioning the cervical nerve roots and this putting excess strain on the brachial plexus. When that happens you will get the numbness or tingling feeling after a certain amount of time. Now, you say your neck and traps are much bigger, therefore the space that these nerve roots have is diminished. Think of it like wearing a boot skiing. After a certain timeframe, the calves and foot start to get numb. Although what you have isn't a compartment syndrome, it's the closest thing I can compare it to. Quick question, do you ever get the numbness throughout the day if you cough/sneeze?

I would try to sleep on your side with enough cervical spine support that your head is in a neutral position. That means in the center, not too high nor too low. Another thing you can do is what I like to call "nerve stretching". Stand in a doorway gripping the door jam. Perform a pec minor stretch, but each time you get the deep stretch, turn your head down and away. This helps stretch both the brachial plexus and nerve roots as well as the pec, traps and sternocleidomastoid.

Hope this help!

Had the same problem a few months ago, but I was traveling for work, and we usually rent out a house in a group and I had the floor with some blankets and Styrofoam mattresses and I would wake up with one of my arms numb ... I blamed it on the floor tho lol... I don't like air mattresses so it was the old school way for me lol

I have to sleep on my stomach with my shoulders up because of previous injuries, this is the only way I'm able to sleep to. Any idea on what could be causing this?

read this. You answered your own question. You have to sleep with your arms up and you are waking up with numb arms...

What was ur previous injuries