In the disease designated by psychiatrists as _dementia praecox_, the patient may become entirely negative, some time regressing to the level of the unborn child, and withdraw even more entirely from reality than the sleeper who, without awaking, is conscious of certain stimuli and performs certain actions showing a comprehension of their nature. CHAPTER II: FATIGUE AND REST What causes sleep? What causes us to withdraw partly our attention from our environment? The answer: brain anaemia, is unsatisfactory for we may ask in turn: what causes brain anaemia? A study of brain anaemia leads one to conclude that it coincides with the usual sleeping period and that it is produced by sleep instead of producing sleep. The large majority of laymen and scientists, however, give a much simpler answer: we go to sleep because we are tired and need rest. Even as sleep and death have been coupled in the literature of all nations, fatigue and sleepiness, rest and sleep have come to be generally considered as synonymous. Fatigue, however, is as difficult to define scientifically as sleep. Drawing a line between physical fatigue and mental fatigue does not simplify the problem; on the contrary, it complicates it by positing it wrongly. 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.