Headaches form as a result of not drinking enough water due to the brain needing a lot of fluids to function. Brain fog and headaches are clear signs that you need more water. A lack of enough fluids causes headaches to form spontaneously. They often persist throughout the day, and often appear to have no known cause. Headaches caused by dehydration are due to a lack of adequate blood and oxygen flow to the brain. The brain then shrinks and pulls away from your skull, causing a headache.