Well from what I know in my limited experience in biology and knowledge, I can deduce a few answers to your question. When a limb goes numb, much of what you are feeling is the depravation of oxygen to your muscles, usually by cutting off blood circulation. The cells send electrical impulses to the nervous system to alert you of the lack of oxygen, which the impulse is registered as the feeling of numbness. The electrical impulses, if great in energy, can be enough to make you move or wake up if you're in a light enough sleep.