I suffered an instance of heat exhaustion when I was in college back in the 80s. I had a summer job working as a playground attendant and on one particularly humid day I over-did it. I felt exhausted and listless, and while riding my bike home it only got worse. I stopped at the volunteer fire hall on my way home because I thought I was having heat stroke. They seemed to think since I came in on a bicycle it must not have been that bad, and kind of blew it off- didn't even offer me anything to drink. I hung out in the AC for a while, took a long drink from the water fountain, and continued home, still feeling terrible. I went home and laid down on the concrete in the basement figuring if my temp was elevated it would be a good heat sink. It seemed to do the trick although it was several days before I felt normal again, and for many years afterward I was not very heat tolerant. (Ironically, my heat tolerance has only gotten better since I've started riding again a few years ago.)