I think much of it is in the mind...Not that it isn't real -- but if you expect to feel fatigue you probably will. In fact, (not that I'm proud of this!) there have been drinking binges with work friends that have lasted until the wee hours, only to go home, get some breakfast then back off to work at 5/6am with no sleep at all. It's really not a problem, though I can't do this too often. And being hung over in work is not a pleasant experience, but worth it for a good night!
As for heat exhaustion, I do get sweaty if I do a lot of physical work... but never exhausted. Maybe I never push myself hard enough? I don't know