We know that there is no purely physical fatigue. Fatigue is only caused in a very restricted measure by the accumulation of “fatigue” products or the depletion of repair stocks.
Under certain “mental” influences, our muscles can perform much more than their usual “stint” without showing fatigue. Hypnotize a man and he will do things he could not attempt in the waking state. He can lie rigid, reposing on nothing but his neck and heels; he can even support in that position the weight of a full-sized man. Men on the march can show wonderful endurance provided their “spirits” are kept up by some form of cheer, band music, etc. Ergograph observations show that signs of muscular fatigue appear and disappear without any obvious “physical” reason. Standardized motions which have been made almost automatic, tire us less than conscious activity.
We shall not deny that in certain cases fatigue may appear purely “physical.” When a continued expenditure of energy, walking, carrying heavy burdens, has induced muscular soreness, the organism must cease exerting itself for a while and recuperate.
But relatively few people perform physical activities which actually wear out the organism.
Even then, if that form of exhaustion was conducive to sleep, the more complete the exhaustion was, the deeper the sleep should be.
Yet we know that people can be “too tired to sleep.”
This is easily explained through a consideration of a phenomenon known as the “second wind” and which, before Cannon’s observations on the chemistry of the emotions, was rather mysterious.
Athletes competing on the running track are often seen to falter and fall back, apparently exhausted; after which, they suddenly seem to breathe more freely, they overcome their limpness and start out on a fresh spurt which may cause them to head off steadier runners.
What happens in such a case is this: great physical exertion causes a form of asphyxiation. Asphyxiation and the concomitant fear, liberate adrenin which restores the tone of tired muscles and also glycogen (sugar) which supplies the body with new fuel.
If the exertion continues long enough to use up all these emergency chemicals, the muscular relaxation necessary for sleep may be obtained. Otherwise, the organism prepared for a struggle with reality, will not lend itself to a flight from it. Although we are “worn out” we toss about in our bed, try all possible sleeping positions and only sleep when the energy which was supplied for a long struggle has been entirely burnt up.