An interesting topic nailit.
Many years ago I found myself in a position in which survival on a mountain was doubtful. When I actually believed I wasn't going to make it I found a deep calm the like of which I have never experienced again. As it happened I was able to get out of it when morning came, frostbitten but alive. Looking back, I think my body got so cold that the mind began to shut down and I now believe that these experiences are largely due to physiological causes