Overheating your body enough to limit athletic performance (whether due to associated dehydration or not) is probably enough to impair the brain as well. Dehydration is known to cause headaches.
I think the effect exists. But what's the size, when you're merely sedentary + thinking + suffering a hot+humid day?
I was very interested in the discussions and opinions that grew out of the last time this was played, but find digging through 800+ comments for a new game to start on the same thread annoying. I also don't want this game ruined by a potential sock puppet (whom ever it may be). So here's a non-sockpuppetiered Irrationality Game, if there's still interest. If there isn't, downvote to oblivion!
The original rules:
Enjoy!