Insufficient water and electrolytes in the body causes the brain to literally shrink. Depending on the severity of dehydration experienced, the intensity of the headache may vary from mild to a very severe migraine-like headache. This may prevent you from going to work.
The shrinking of the brain causes to detach from the skull in some areas, which causes pain. Once the fluid balance is re-established, the brain will return to its normal size, and the headache should disappear.
Severe dehydration, followed by the loss of more than 10% of fluid volume may provoke seizures. Seizures are potentially life-threatening conditions and you should seek immediate medical attention. People who have already experienced seizures, either due to epilepsy, or other brain disorders are more prone to seizures if they get severely dehydrated.