We have not yet exhausted all the theories, nor shown how much too much some of them and how far too little all of them prove.

The two remaining scientific theories of sleep are the psychological and the biological. The best modern exponent of the psychological theory is Marie de ManacÉÏne, who defines sleep as “the resting-time of consciousness.” Persons whose consciousness is but little developed, young children, and those of weak intellect, usually require a great deal of sleep, while persons whose minds are active, alert, responsive, get along with comparatively little sleep.

For a long time it was believed that living creatures devoid of consciousness would not sleep at all, but recent experiments have apparently weakened that conclusion. Dogs, pigeons, and other animals deprived of brains in the interest of scientific discovery, appear to sleep, that is, they have periods of inactivity, just as those with brains and consciousness. Belmondo, after repeated experiments, drew the conclusion that sleep is not a purely cerebral function, as some believe, but that the whole organism sleeps; and the brain sleeps only because the organs of sense sleep. This, however, is doubtful.

And this is in sum and substance the biological theory of sleep, that the whole organism sleeps, but even here there are exceptions. It is true that the heart beats less rapidly; that we breathe less frequently; that the brain cells cease their functioning because the neuroglia contracts and shuts off or lessens their joint activity; the motor consciousness rests; the nerves of sensation refuse to be stimulated, we sleep. Yet we know that the spinal cord never sleeps, that certain functions of the body continue uninterruptedly in the sleeping-state as in the waking-state, and, after all these years of theorizing and experimenting, we do not know definitely what sleep is. We know the mechanism of sleep, its manifestations and its effects; we know that continued sleeplessness means madness and death; that sleep is essential to the physical and mental well-being of the human organism, but we do not know what sleep is any more than we know what life is. There is a limit to what material science can know.

Heubel, who is one of the recent supporters of the psychological theory of sleep, says that “Mental activity depends on the incoming peripheral sensory stimulations; when such peripheral sensory stimulations are absent, mental activity is in abeyance and sleep results.” This is, in effect, to say that, when things about us no longer give us any sensation, when they do not attract or hold our attention, we fall asleep. But we all know of exceptions to this rule. We have seen others fall into “a brown study,” and have probably done so ourselves, and become perfectly lost to all about them; absorbed in their own reflections, they neither hear nor see the things happening around them. For the time being “peripheral sensory stimulations” are absent, and yet mental activity continues and sleep does not result.

The biological theory of sleep considers all the other theories, while formulating its own, because biology considers the whole organism and not only one organ or function of the body. From a different point of view Binns’ theory is confirmed by ClaperÈde, who points out that, “biologically regarded, sleep has its significance not as a passive state, but as an active instinct, like all the other instincts of animal life.” There is a degree of satisfaction to be found in this theory. It might be stated in this way, that, when man has had during any period all the sensations and experiences he can digest, the instinct to sleep takes possession of him. It is not that he becomes helpless in the hands of those experiences, but that his whole nature, like his stomach, knows when it has had food enough, and desires time for digestion and assimilation before it takes in more. Obviously, “utter separation from the phenomenal world,” as John Bigelow expresses it, becomes necessary.

In his lectures on Botany at the Royal School of Medicine in Great Britain, Professor Leo H. Grindon said: “Man is captured in sleep not by death but by his better nature; to-day runs in through a deeper day to become the parent of to-morrow, and to issue every morning, bright as the morning of life, and of life-size, from the peaceful womb of the cerebellum.” This is the result not of a passive state, but of an active instinct; it accepts sleep as a time of growth, not merely a time of rest. Bigelow says, “Something goes on during sleep which is a preventive as well as an antidote to mania,” and, in furtherance of this same idea, Dr. J. J. G. Wilkinson of the Royal College of Surgeons of London, argues that it seems “as if a reason more perfect than reason, and uninfluenced by its partialities, had been at work when we were in our beds.” Even “biologically regarded,” Bigelow is not far astray when he claims that “our desire for sleep is manifestly designed to promote in us the growth and development of spiritual graces.” In other words, we desire to sleep, that we may relate the experiences of our every-day active life to the sum of knowledge we already possess by inheritance and past experience, that we may thereby get a fuller understanding of life and its purposes.

“It is not uncommon for those who have no habit or inclination to sleep during the morning hours of secular days, to be overcome with somnolency in church soon after the devotional exercises are begun, and to find it impossible to derive any edification from them until they have lost themselves for a moment or two in absolute unconsciousness. Then they have no difficulty, sometimes a lively pleasure, in attending to the exercises which follow. The worshiper is then withdrawn from the familiar excitement of customary avocations. It is idle to suppose that in these few moments of repose, upright in his pew, he has rested enough, in the common acceptation of that word, to repair any waste of tissue that would explain the new sense of refreshment that ensues. He has received, in that brief retirement from the world, some reinforcements which manifestly are not dependent upon time or space for their efficacy—spiritual reinforcements, and spiritual reinforcements only. He has removed himself, or been removed, further away, out of sight or hearing or thinking, so to speak, of his phenomenal life, and nearer to the Source of all life.” This explanation may or may not be true. He adds:

“It was quite a common impression among the ancients that sleepers in temples of religion were more apt to receive divine communications there than elsewhere.” (“Mystery of Sleep,” John Bigelow, pp. 94-95.)