Indeed, it is not too much to say that headache is the most useful pain in the world.
It has little to do with the condition of the brain, but occurs in the head chiefly because the nerves of the head and face are the most sensitive of all those in the body, and the first ones, therefore, to "cry out" when hurt.
Headache has been described as the cry of a poisoned or starved or over-worked nerve, and is simply nature's signal that something is going wrong.

Toxins, or poisons, formed anywhere in the body, from any cause, get into the blood, are carried to the sensitive nerves of the head and face, and irritate them so that they ache.

It is foolish to try to do anything to the head itself for the relief of headache, although cold cloths, or a hot-water bottle, may be soothing in mild cases.

The thing to do is to clear the poison out of the blood, and the only way is to find what has caused it.
Nearly all the things that cause headache do so by poisoning the blood.
A very common cause of headache, for instance, is getting over-tired, especially if at the same time you do not get enough sleep; and, as you already know, tiredness, or fatigue, is a form of self-poisoning.
Another very common cause of headache is bad air--sitting or sleeping in hot, stuffy rooms with the windows shut tight.

If you do this, not only are you not getting oxygen enough into your blood to burn up the waste poisons that your own cells are making all the time, but also you are breathing in the waste poisons from other people's lungs, and the germs that are always in bad air.
Another very common cause of headache is _eye-strain_.