I ride in the heat and cold. For heat, I wear the besting wicking clothing I have, hydrate fully at least 1/2 hour before riding, take lots of fluid, and moderate my ride routes. I've learned which parts of which rides are shady or exposed to full sun at which times of day. I also tend to moderate the intensity of my rides. If, I'm feeling tired, I back off a bit. To me being outside riding, even at a slower pace, is much better than being inside on the trainer with boredom driving me nuts.