Extreme Southwest Virginia (U.S.A) here with quite a few recent 95+ degree days (and close to 100% humidity) ...Consider taking a break every twenty minutes.

The heat really got to me about a month ago while I was mowing with a gas push-mower and trying to get the mowing done as quick as possible (I usually have no problem mowing 45-60 minutes strait through without taking any breaks or consuming fluids). I guess that's the closest I came to having "heat stroke"...At one point I could feel my heart beating like I had just ran a sprint and I actually felt kind of cold, so I really slowed down (I did have a wet towel around my neck that I used to wipe my face and head, which did seem to help). Of course I should have stopped at that point to take a break and drink fluids, but I finished the last 15 minutes of the mowing, went inside and sat down. I felt pretty ill for around 30-60 minutes. I didn't "black-out" or anything, I mostly felt "not right," like maybe I was going to start experiencing worse symptoms. The next time I mowed I "made" myself take a break after 20 minutes ( I went inside, sat down, drank water, and didn't go back out until I felt cooled down, which took about 10 minutes).

During this period of great heat (within the last month, and after the mowing incident) the closest I came to experiencing a similar "heat exhaustion" on my bike was when I tried to "power" my way up a hill (and the hill was not very big really). As with my mowing experience I was trying to get the climb over as quick as possible (my thought process was "If I get up this hill fast this climb will be behind me and I won't have to worry about it anymore"). By the time I was 3/4 the way up the hill I was feeling over heated. I'm pretty sure I got the heat-induced high heartbeat rate along with the higher heartbeat rate one would normally expect with exercise, but at least I was drinking water this time and I recovered okay on the decent. On the next hill, which seemed to be much bigger, I climbed as SLOWLY as possible and I never got that overheated feeling. Like someone already wrote, in this heat it is about survival over speed when getting to your destination.

I know sometimes it might be best to keep riding and keep a breeze going (as opposed to sitting under the sun and just roasting) but if you find a bit of shade to sit in (like every 20 minutes) that can keep you from overheating. Someone else made a good point about listening to your body and not ignoring warning signs. Some people (like me) might have to fight their impulses to just "power through" an activity in this heat. From my experience I will watch for these symptoms: a high heart rate ( I mean feeling it pounding in my chest) for no good reason, feeling cool (maybe it would be more accurate to call it "clammy"), and/or basically feeling overwhelmed by the heat and not being able to recover and get back to feeling pretty normal (or as normal as you can feel while soaked in sweat).