Hi there, I live in Chile at the moment. In summer, we hit around 100 degrees and above.
Last summer (which is Jan, Feb here) I drove from Santiago to Puerto Montt. It’s a little over 1000 Kmh’s.
I never got a headache. Sure, with so much heat hitting your neck, shoulders and helmet…. it is obvious that the fatigue hits you faster than it would normally.
I stopped around 3 times (or maybe 4) for refueling, and each time I had to take a few minutes extra just to take on liquid myself. To relax, stretch my legs, and get rid of the dizzy feeling I had. It’s not the helmet itself, just your body struggling with heat exhaustion.
You “might” want to try a larger helmet if you want, but remember that it isn’t supposed to be moving around your head. It is supposed to be quite tight… it is trying to save your life.
Good luck!