There is another class of investigators who aim to explain what might be called the nervous mechanism of sleep rather than its causes. These are the histologists, and theirs is the “histological” theory of sleep. There is a vast literature on this one phase of sleep theorizing alone, and it is deeply interesting to those who, in order to understand this theory, are willing to wrestle with the difficult and technical terms.

The general reader, unfamiliar with physiological terms, is bewildered by such a word as “neuroglia.” He wonders what sort of a fossil that is, when in fact it is merely a particular sort of tissue found in the central nervous system; a substance without any nervous property serving a purpose merely similar to cement. So that, after all, like much science, it is simple enough when put in plain words.

Some histologists hold that the neuroglia is able to contract or expand; that, when expanded, it takes or receives impressions from without, and, when it contracts, it shuts out such impressions, thus inducing sleep. Dr. J. Leonard Corning, of New York, says that “Sleep may be defined as that state of the central nervous system in which the higher centers are, to a great extent, in a condition of physiological quiescence, with all the consequences thereby implied.” Dr. Corning means that the nervous centers of the brain are inactive, as a result of contraction, and that this state results in drowsiness and in consequent loss of the consciousness. He recognizes, however, that this purely physical condition does not always produce sleep; that there may be disturbing causes within. He says: “Those who suffer from sleeplessness are, almost without exception, beset by a variety of disagreeable mental symptoms during the day, dread of impending evil, irritability, depression, dread of society, etc.” Although these are often the result of wrong states of mind or heart, he recommends for such cases warm baths, Turkish baths, massage, and for obstinate cases he even suggests the use of drugs, because he regards the formation of habits of insomnia as more likely than the formation of the drug habit. This suggestion is not generally favored by the investigators of sleep, the most of whom discountenance the use of drugs. Almost everyone has known somebody who contracted the “drug habit,” or has heard of somebody who died from the effects of an overdose of some poison taken to induce sleep. Nobody should dose himself with drugs, hoping to get good results from sleep thus secured. It is wiser by far to discover the cause of sleeplessness and remove it, rather than merely to stupefy ourselves for the time being.

It was as a worker along histological lines that Henry Hubbard Foster of Cornell University became convinced that sleep is induced by the absence of stimuli: that is, of things that attract and hold our attention. It may be that the individual withdraws from all stimulating conditions and creates conditions to cause sleep, as we do when we prepare for bed; or it may be that, because of fatigue, our senses do not respond to the things that would otherwise stimulate us. In either event, the result is the same—there is an absence of stimuli.

Foster believes that the present state of our development is not sufficient to meet the demands of continuous activity of the senses and the brain. “If it were not for fatigue,” he says, “the development of the nervous system might be carried to such a point that consciousness could be present continuously.” He finds the reason for sleep in “a temporary derangement of the nervous system.” According to Boris Sidis of Harvard, who has experimented extensively on frogs, cats, birds, dogs, children, and adults, the cells of the central nervous system, by expanding and contracting, connect themselves with, or cut themselves off from the whole nervous system, and induce “waking-states and sleeping-states.” The purely scientific man is forever aiming to reduce all the phenomena of human life to a simple formula. But no formula has yet been discovered which includes all phases of life to the satisfaction of all its students. Hence we have so many different theories of so natural and universal a function as sleep, none of them perfectly satisfactory even to their discoverers or inventors, and none affording any great help to those who want to know how to sleep.

This whole neuron theory, as it is called, of dilating and contracting is really no more complete an explanation than any of the others. No perfect explanation of any natural function can be given until we can fully explain life. That has not yet been done. The most advanced biologists can say, “Here life appears,” but they cannot absolutely define life any more than they can create it out of inanimate things.

We know, however, that in sleep, sight, nearly useless to man in the gloom of night, goes first to rest. Next taste goes. Then smell, still so useful to animals, deserts us. Then touch is dulled. Last of all, the hearing relaxes its guard, though with some persons it stays long awake. Noise will arouse us quicker than a touch; and last the light.

As you drop off to sleep you can notice the decreased sensation in the long-serving feet which feels as if it slowly climbs to the muscles in the head and neck.