Yes, I do find that heat can really affect me, too. I have to be very careful to stay hydrated, and I usually do rest a little bit more when it's very hot. I also find that my eyes can really be bothered by being out in the bright sunlight for too long, so I now make sure I always have my prescription sunglasses on hand!
Summer is still my favorite season, though! IIH won't take that away from me.