What exactly makes your arm or leg fall asleep? I’ve always wondered that. I’ve got a damaged nerve in my upper spine that cause my left arm to go numb and fall asleep at odd moments. But even before that injury I’ve wanted to know that.
Interruption of blood supply to the relevant nerves, I believe.
http://www.madsci.org/posts/archives/dec96/850996021.Ns.r.html