4) Your head is pounding.
Experts don’t know exactly why dehydration can cause a splitting headache, according to Dr. McStay. But they do know almost all bodily systems – including the brain, nerves, and sinuses, which can contribute to certain kinds of headaches – rely on the appropriate balance of water to function correctly. “When electrolyte levels suddenly shift, the body can’t always compensate,” Dr. McStay says by way of explanation.