Once, however, we have clearly recognized that no headache should be treated too lightly or indifferently, it may be frankly admitted that practically the vast majority of headaches in which we are keenly interested--that is, the kind that we individually or the members of our family habitually indulge in--do form a moderately uniform class among the hundreds of varieties, and are in the main due to some six or seven great groups of causes. We have learned by repeated and unpleasant experience that they are very apt to "come on" in about a certain way, after a certain set of circumstances; that they last about so long, that they are made worse by such and such things, that they are helped by other things, and that they generally get better after a good night's sleep. One of the commonest causes of this group of recurrent and self-limited headaches is fatigue, whether bodily, mental, or emotional. This was long an apparent stumbling-block in the way of a poison theory of headache, but now it is one of its best illustrations. Physiologists years ago discovered that what produced not merely the sensation but also the fact of fatigue, or tiredness, was the accumulation in the muscles or nerves of the waste-products of their own activities. Simply washing these out with a salt solution would start the utterly fatigued muscle contracting again, without any fresh nourishment or even period for rest. It has become an axiom with physiologists that fatigue is simply a form of self-poisoning, or, as they sonorously phrase it, autointoxication. One of the reasons why we are so easily fatigued when we are already ill, or, as we say, "out of sorts," is that our tissues are already so saturated with waste-products or other poisons that the slightest addition of the fatigue poisons is enough to overwhelm them. This also explains why our pet variety of headache, which we may have clearly recognized to be due to overwork or overstrain of some sort, whether with eye, brain, or muscles, is so much more easily brought on by such comparatively small amounts of over-exertion whenever we are already below par and out of sorts. People who are "born tired," who are neurasthenic and easily fatigued and "ached," are probably in a chronic state of self-poisoning due to some defect in their body-chemistry. Further, the somewhat greater frequency and acuteness of headache in brain workers--although the difference between them and muscle workers in this regard has been exaggerated--is probably due in part to the greater sensitiveness of their nerves; but more so to the curious fact, discovered in careful experiments upon the nervous system, that the fatigue products of the nerve-cells are the deadliest and most powerful poisons produced in the body. Hence some brain workers can work only a few half-hours a day, or even minutes at a time; for instance, Darwin, Spencer, and Descartes.