Bill, during a winter class, and particularly with a winter Hell Week, the cold will typically be the reason most candidates cite for ringing out (yeah, it does get that bad; especially when the sun goes down).
Anecdotally, surfer boys are said to cope with this misery the best, as well as the dudes who managed to somehow find the sweet spot between high mileage preparation and maintaining (what they claim) an insulting layer of blubber.