Dehydration means one thing: You're losing more water than you're taking in. In summer, it's hotter and most of us are more active. If you sweat and neglect to replenish the fluids you lose through perspiration, you could be dehydrated, which can lead to a headache.
HEAD IT OFF: Drink at least eight glasses of water a day—more if you're very active. If you still get a headache, drink 16 ounces of water, and rest for 15 minutes. If dehydration is the culprit, you should feel relief shortly.