For probably about the past year or so, when I fall asleep at night when sleeping on my stomach with an arm above my head bent 90 degrees to sleep on top of, I wake up either in the middle of the night or in the morning not being able to move my arm. When it first started I would not be able to move my arm whatsoever for a few minutes until blood circulation restarted, but I think in the past 6 months or so I've gotten so used to it that I wake up in the middle of the night as soon as I feel it and reposition my arm after I get the blood flowing again. Is this a sign of something more serious like diabetes? Carpal tunnel syndrome? Or is it the result of working out more and now the nerves are pinched when sleeping more than they used to be? It feels as though the nerve that usually is sensitive to pressure is under my arm, near the arm pit except on the underarm side, rather than the side of chest side of the armpit. Usually no problem when awake I don't think.

What kind of work or hobbies do you do?
desk, repetitive?? upperbody posture - head/shoulders forward??
How much & often do you lift weights?
Stomach sleeping is about the worst position for your back, you might want to search about it online also.

You may want to visit out Thoracic outlet syndrome forum (TOS for short) we have had weight lifters join this last couple of years due to similar issues.
http://neurotalk.psychcentral.com/forum24.html

be sure to check out the sticky threads at the top of that page