It's the sun!
I'm a fairly heat tolerant person. I grew up in the South, it gets disgusting, sticky, oppressive hot down there. Heat and humidity are my friends. It's the sun I can't stand.
It doesn't help that I'm also very fair skinned. I have no protective layer of melanin to protect me, I just turn red and miserable as the sun cooks my innards like a microwave.
Spider, on the other hand, seems utterly unaffected by the heat. Which is a good thing, but it's also really, really annoying. I'm starting him back into work slowly after our extended, heat induced hiatus. Currently, we're working every other day and just the basics: forward, round, relaxed. He doesn't really get "slowly", though. Thoroughbreds just don't do "slow". They're not so good at "relaxed", either.
Because it's still hot (with no end in sight), I've taken a cue from from other animals and become crepuscular. That means active at dawn and dusk. It's a fun word to say. Try throwing it into your next normal conversation and see what kind of reactions you get.
Normal friend: "Hey, how 'bout this crazy weather?!"
You: "Yeah, I've totally become crepuscular"
*watch with glee as normal friend splutters and tries to figure out how to end this awkward conversation in a dignified way*