It doesn't take cold weather to get hypothermia. There have been documented cases of people succumbing to the effect of hypothermia when the temps were in the high 60's to low 70's. This occurs quite often with adventure racers where after a few days of punishing conditions, extreme physical demands, lack of sleep and lack of proper nutrients / fluids, their bodies are unable to produce the energy required to maintain body temperature.

Back to the OP"s question. You just don't simply fall asleep and not wake up. Susan's post pretty well explains it and the hypothermia article on wikipedia also has some good general info.