What causes a dehydration headache?

What causes a dehydration headache?

When you’re dehydrated, your brain and other tissues in your body shrink (contract). As your brain shrinks, it pulls away from the skull, puts pressure on nerves and causes pain.

Even mild dehydration can lead to a headache. When you drink water and other fluids, the brain plumps up to its previous size and the pain goes away.