Based on my experience in those conditions, you might be low in fluids and perhaps salt (aka electrolytes).
Try to weigh yourself before you roll out in the morning, and again when you get home. If you lost weight, it's water weight. Using myself as an example, I can sweat twice as much as I can drink in a hour; multiply that by 4-6 hours in the heat, and I can easily end up a gallon low after a long ride. That's plenty to cause headaches. Of course, if you drink too much and don't balance it with salt, that can cause headaches too. IIRC you need about 3 grams of salt per quart (liter), so that's 12 grams of salt to balance that gallon I need to drink. It's a lot. A baked potato with twice as much salt as any reasonable person would put on it, or a large bowl of pho, will start to make a dent in the salt deficit.
And force fluids at night. The three times you need to drink are: (1) any time you think about drinking; (2) any time you're not thinking about drinking; and (3) in between.