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.

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.The majority of people, after all, busy themselves with tasks which do not really deplete their stores of energy, but which prove monotonous. That monotony is then interpreted as fatigue.

In such cases, rest seems to be more easily attained through a change of activity than through mere cessation of activity.

A business man has been closeted in his office attending to many tedious details, reading letters and answering them, etc., and by five o’clock he feels “tired.” He will then go home, change his day suit for evening wear, attend a dinner at which he will do perhaps much talking, then watch actors for three hours and feel “rested.”

Or at the end of a “heavy” week, he will gather up his golf outfit and walk miles in the wake of a rubber ball. He returns to his work “rested,” although he has only exchanged one form of activity for other forms of activity. Of actual “rest” he has had none.

Children “tired” of sitting in a class room will romp wildly, shout at the tops of their lungs, jostle and fight one another and return to meet their teacher “rested.”

Undirected activity in the young, pleasurable activity in the adult do not seem to make rest necessary, and in fact are a form of “rest.”Egotistical gratification easily takes the place of rest. Heads of large businesses have sometimes mentioned to me that they worked much harder than some of their employÉs. Some of them kept on revolving commercial schemes in their heads or attending business meetings long after their office workers had left. “And yet,” they added, “we are not complaining about being tired.” Nor were they as tired, after fifteen hours of “free labor” as their employÉs were after six or eight hours of routine work allowing them very little initiative and independence of action.

Edison works eighteen hours a day and only “rests” through sleep some four hours out of the twenty four. I wager that if he were put at work in his own plant, under the direction of a foreman, performing regular, monotonous tasks, he would break down under the strain of such long hours and would have to “rest” twice as much as he does now. His work satisfies him, and every new detail he perfects, every novelty he initiates, vouchsafes him a powerful ego gratification.

Napoleon, too, could perform incredible feats of muscular activity and endurance after which four hours’ sleep were sufficient to rest him. His life was for many years a continuous round of ego gratifications, won at the cost of great exertions, it is true, but proclaiming to him and the world his almost unrestricted power and luck.

One is forced to the conclusion that a desire for rest is a desire, not for decreased activity but for increased activity.

I shall make this point clear through a simile. The manufacturer who “attends to business” must, in order to succeed, “concentrate” on a few subjects and exclude all others from his mind. He may for a few hours think of nothing but, let us say, a certain grade of woollens, certain machinery, a certain customer and perhaps a certain engineer and some financial problem connected with those four thoughts. He must therefore exclude from his mind at the time, thoughts of playing golf, buying new clothes, going to the theatre, renting an apartment, repairing his motor car, thoughts of meals, women, card playing, and many other thoughts which are clamouring for admission to consciousness because they all represent human cravings.

In his relaxed moments he will let all those other thoughts come to the surface. Which means that, what tired him, was the fact that he had to keep all those subjects down and allow only the other four to rise to consciousness.

Mental rest consists in admitting ideas pell mell into consciousness without exercising any censorship on them. It consists in passing from a reduced but directed mental activity to an increased but undirected mental activity.

In other words, rest is the free, normal, unimpeded functioning of the vagotonic nerves which upbuild the body and assure the continuance of the race. Ego and sex activities, mental and physical, are constantly struggling for admission to consciousness and for their gratification. They are held down, however, by the sympathetic nerves which play the part of a safety device, moderating or inhibiting the vagotonic activities whenever the latter might endanger the personality.

Physical and mental rest, however, being easily attained through a change of activities, cannot be entirely synonymous with sleep. Sleep takes place mainly while we are resting, although we know of cases when sleep sets in regardless of continued muscular activity, but sleep is not exactly “rest.” We do not sleep because we need rest. In many cases we can or could rest very well, although in such cases sleep is an impossibility.

What then induces sleep? The certainty that we can for a time relax our watch on our environment; a feeling of perfect safety; the conscious or unconscious knowledge that no danger threatens us.

Our receptive contact with reality is attained through the action of our vagotonic nerves which, as stated before, upbuild the body and assure the continuance of the race. Our defensive contact, on the other hand is attained through our sympathetic nerves which interrupt all the activities which are not necessary for fight or flight. As long as some stimulus is interpreted by those nerves as indicating a possible danger, we cannot sleep, although we may, under the influence of terrifying fear, fall into unconsciousness.

A light flashed on our closed lids at night causes us to wake up because sympathetic activities bid us to prepare for an emergency. A light burning evenly in our bedroom and not too bright to cause physical pain, will, on the other hand, allow us to sleep soundly because the constant character of the stimulus does not cause us to expect any danger therefrom.

A mouse rustling a bit of paper will wake us up, but trains passing in front of our window at regular intervals, or the constant rumble of a neighbouring power house will not prove a disturbance as soon as our nerves have learnt to interpret those stimuli as harmless.Conversation with a dull, witless person, unlikely to best us in debate, puts us to sleep. Argument with keen, sharp-minded people, who keep us on the defensive, may lead to sleeplessness for the rest of the night. A dull book in which nothing happens or is expected to happen, acts as a soporific; we cannot close our eyes before we know the dÉnouement of a thrilling piece of fiction.

In other words, monotony transforms itself into a symbol of safety. Safety does not require the muscular tension, the blood stream speed which the organism needs in order to cope with possible emergencies. We “let go” and no longer pay any close attention to our environment. We sleep.