4. Headache
One of the most common signs of dehydration is a headache. This can be distracting and painful but it’s even less pleasant once you know what’s actually going on. You see, your brain contains a lot of fluid and when you’re dehydrated, you lose a lot of this fluid.
That in turn means that your brain can start to contract and shrink, just as a sponge does when it has no fluid. This causes the brain to pull away from the inner skull where it is attached, resulting in pain.