Hi Melissa. I got that numb arm all the time and for a very long time. I was told to sleep on my back but never could achieve success until just now. I FIANLLY changed to sleeping on my back and now I don't wake w/numb arms.
This was the key for me--
-a wedge under my lower legs keeps me comfortably flat on my back
-a smaller tempurpedic pillow for the correct neck postion
-some rolled up and taped bubble wrap placed in between my shoulder blades resting vertically right under the pillow

I had a PT help me figure it all out. And dr Newkirk invented the bubble wrap idea but his was 2 rolls; one under the neck horiz and the other vertically just below it. for me the tempurpedic works much better. but none of it works for me w/o getting the knees up. It keeps me from turning onto my side and keeps me comfortable. I haven't invested in a proper one yet but I'm working on it.