An enterprising journalist has recently published the replies of a number of well-known men to an inquiry as to how many hours’ sleep they are in the habit of taking, and what they find to be the best remedy for sleeplessness. Such an inquiry naturally leads on to further thoughts about “Sleep.” What a mysterious, yet sweet and lovable thing it is! How strange it is that we all regularly and gladly abandon ourselves to it! How terrible is the state of those who cannot do so! And then one is led to ask, what is it? and why is it? Do all living things sleep for some part of the twenty-four hours? How does it differ from mere resting, and in what does its virtue consist?

Shakespeare has said the most beautiful words that have ever been uttered about sleep, and that because he knew what it was to seek for it in vain—

And again, when the strenuous life of the great Bolingbroke has at last overtaxed his brain, and he can no more find rest and unconsciousness at night, Shakespeare makes him say—

Poets have as a rule been too ready to make much of the likeness of sleep and death, whereas there is an absolute difference in their mere appearance. Sleep makes even those who are ill-favoured and coarse look beautiful, imparts to its subjects a graciousness of expression and of colour, and a gentle rhythmic movement, whilst suffusing them as it were with an “aura” of contented trustfulness. These things are far from the cold stillness of pallid death. And this depends upon the fact that in sleep, though many of the activities of the body and mind are checked, and even arrested, there are yet still present the never-ceasing pulse of the heart, the flow of the blood, the intake and output of the breath, and a certain subdued but still active tension of muscles, so that though the body and limbs are relaxed they never assume the aspect of complete mechanical collapse which we see in death. The pupils of the eyes are strongly contracted during sleep, not relaxed and expanded as are those of wide-awake people in the dark. There are some well-known works of art—both painting and sculpture—in which the dead are not truly represented, but are made to retain the resistfulness and pose of living men and women; others show true observation in presenting the startling and distinctive flaccidity of the newly dead, which is followed after a few hours by the equally characteristic rigor mortis, or stiffness of the dead. There are many fine studies of sleep by sculptors, but none which to my thinking so delicately and truthfully present its most beautiful and peculiar effects on the muscular “tone” as a work in the Luxembourg Gallery in Paris, called “Le Nid”—a baby of a year old and a little girl of three or four years, asleep side by side on the cushion of a capacious arm-chair. The pose and the details of muscular relaxation differ greatly and characteristically in the two children. One would like to see sleep at different ages and under various conditions of fatigue similarly portrayed, for there is a range and variety of expression in those who sleep, not perhaps as extensive, but as beautiful as that to be found in those who are awake.

All things on the earth may be said (if we use the term in a wide sense) to sleep, for all are affected by the stimulation to activity caused by sunlight and by its cessation during night. It is only of late years that we have come to know of fishes, crabs, worms, and star-fishes (many of them without eyes) which live in the depths of the ocean, where no light penetrates and it is always night. The ultimate source of their food is in the upper sunlit layers of water, to which they never penetrate, and from which particles of dead but nutritious matter (the bodies of those who have lived up there) rain down upon them incessantly, like manna on the Israelites. All things accessible to the sun’s rays are not equally, nor even similarly, affected by the alternation of day and night, and some not directly at all, but only by the sleeping and waking of other things. The food of all living things comes ultimately from plants which, in the presence of sunlight, and only in that presence, and in virtue of its action upon their green leaves, manufacture starch and sugar from the carbonic acid which exists in the air and water around them, whilst they are also thus enabled to take up nitrogen, and so to form their living substance or protoplasm. At night those particles or cells of the living protoplasm of plants which are furnished with transparent green granules, so as to entangle the sunlight, and by its aid feed on carbonic acid, cease this work. They necessarily repose from their labour because the light has gone. This is the simplest example of the sleep of living things. And that here, too, as in higher creatures, sleep is not a merely negative thing—a mere cessation—is shown by the fact that it is at night that other changes go on in the plant. The manufactured food takes effect on the cells or particles nourished by it; in the night the well-fed, enlarged “cells” in the growing parts of many plants slowly divide each one into two, and each of these again into two, and so on, so as to increase their total number and produce growth and development of the plant. This alternation of activities in day and night occurs even in the invisible microscopic vegetation of pools and streams. Animals—even the most minute, only visible with a strong microscope—move about in search of “bits” of food—in fact, bits of other animals or of plants—and they, too, are, with special exceptions, checked in their search for food by the darkness, for even extremely minute and simple animals are guided in their search by light—that is to say, by a more or less efficient sense of sight. Thus we see that in a general way the sun is truly the ruler of life, and that when he is hidden from us we all become quiescent, a condition which may be rightly considered as the elementary form—the simplest equivalent of the sleep of man. The quiescence which falls on the earth with the setting of the sun has, however, become the opportunity of two different classes of living things to seize an advantage. Beasts of prey, many of them, sleep during the day, and steal forth at night on velvet foot to pounce on the slumbering animals which are their necessary food. Another group of timid animals, moths and small beasts like mice, hedgehogs, and lemurs, find their safety in the dark, and only then venture forth. Even so, the moths are met by special nocturnal enemies, the bats. So that the primitive arrangement is complicated by a wakefulness, exchanging day for night.

It is natural to apply the word “sleep” to the state of profound repose which other living things appear to enter upon at night, so far as we can judge by changes of activity and attitude—although it must be remembered that the sleep of man is what we really indicate by that word, and that it is difficult to trace anything beyond a superficial similarity between man’s sleep and the repose or quiescence following upon activity in other living things—excepting those which by their structure and the working of their mechanism are obviously comparable to man, such as beasts, birds, reptiles, and fishes. The “sleep of plants” is the term applied to the closing of the flower, the drooping of the flower-head and of the leaves of many of the common flowering plants, which occurs at sunset or during the later hours of sunlight. But it seems that this is not really comparable to man’s sleep. The closing of the flower appears to be a protection of its perfume from useless evaporation during the darkness, and the drooping a device to avoid the settlement of dew and the injurious action of cold. Living things always furnish us with examples of adaptations resisting the general law—and as there are moths which fly by night, so also there are flowers which remain closed by day and open at night to attract these moths, by whom their pollen is carried and their fertilisation effected. The tobacco-plants of our gardens are examples of these night-opening flowers, which attract the nocturnal moths by their heavy perfume, and there are many others.

The movements of plants are much more definite and varied than one is apt to suppose. Leaves and flowers turn to or away from the sun, or to or from the position which will favour a deposit of moisture; or, again, their tendrils will explore and seize upon supports, enabling them to secure a hold, and so to climb. The sensitive plant exhibits rapid drooping movements of its leaflets and leaf-stalks when touched or subjected to vibration.

An allied plant which shows slower but definite movement of its leaflets has been supposed to furnish thereby prophetic indications of the weather, and even to foretell earthquakes. This plant is the Abrus precatorius, the seeds of which are called crab’s-eyes, and are used in India by jewellers and druggists as weights—averaging a little less than two grains. They are harmless when eaten, but contain a poison called abrine, which causes them rapidly to produce fatal results when introduced beneath the skin. Under the name “jequerity” they were introduced into this country in 1882 for the treatment of ophthalmia. This is the plant which was celebrated, about twenty years ago, as the earthquake plant or weather plant, owing to the statements of an Austrian naturalist as to its marvellous powers of prophecy by the movement of its leaflets—statements which were carefully examined by botanists at Kew Gardens at the time and shown to be devoid of justification. Earth tremors, like other vibrations, cause the leaflets to move and change their pose as they may cause animals to utter cries of alarm, but the movements of the leaflets have no more prophetic character than have those of the delicate pendulums, called seismographs, by which it is now usual to register the constantly occurring slight vibrations of the earth’s crust.

That beasts and birds enjoy a nocturnal sleep similar to that of man, which is occasionally—like his sleep—transferred from night to daytime, is a matter of common knowledge. These animals, like man, lower the eyelids and adopt a position of ease when sleeping, even though they often remain poised on their legs. The question has been raised as to whether fishes sleep, since they have no eyelids and remain when at rest poised in the water. We made some inquiries on this subject in the laboratory of the Marine Biological Association at Plymouth some years ago, and came to the conclusion, from the observation of various marine fishes in the aquarium there, that fishes do sleep at night. They come to rest on the bottom of the tanks, and are not so quickly responsive to a touch or intrusion of any kind as they are in the daytime. It is probable that this condition of repose is more definitely marked in some kinds of fishes than in others, but in all shallow-water marine organisms the absence of light produces a corresponding period of quiescence. That there is a good deal more than this involved in the sleep of the higher animals and of man will be apparent when we come to study it more closely.

The sleep of man, and of animals which have, like man, a large and well-developed nervous system—has for its salient feature the cessation or extreme lowering of the “psychical” activity of the brain. When sleep is at its height external agents (such as a touch, a sound, a flash of light) which in the waking state set up through the nerves of the organs of the senses complex changes in the brain, no longer do so. They not only fail to excite consciousness and to leave their mark on the memory, but they do not produce even a simple unconscious response. Yet if they are of a sufficient degree of violence (varying according to the depth of the sleep), they do reach the brain, and thus “awake” the sleeper. Corresponding to the absence of receptive activity of the brain in sleep is the absence of outgoing impulses from that organ; there is no such control of the muscles as in the waking state, the head nods, the eyelids droop, and the muscular action by which the erect posture is maintained is in abeyance, although in a greatly lessened degree some amount of muscular tone is unconsciously retained.

The passage from the waking state to that of deep sleep is not sudden but graduated, and so is the process of awakening. In the intermediate condition, either before or after deep sleep (often only a minute or two in duration) the brain can still receive, more or less confusedly, impressions from the exterior through the organs of sense, and it is in this way that “dreams” are set going, and may be afterwards either forgotten or remembered. In full sleep the mind is a blank. As a rule healthy sleep becomes gradually more complete in the first hour, and then very slowly less profound. But there are not any sufficient observations on the “quality” of sleep after short or long duration. In sleep it is not only the brain which is at rest: the whole body shares in the condition. The pulse and breathing are slower, the digestive organs and the bladder are more or less at rest. Both the intake of oxygen into the lungs and the expiration of carbonic acid are lessened. The chemical changes within the body are lessened though still proceeding, and as a consequence the temperature is lowered.

It is curious how incomplete at present is the physiologist’s knowledge of both the actual condition of the brain in sleep and of the immediate causes which produce that condition. It is probably true (though it is disputed) that the brain becomes pale during sleep, owing to a contraction of the blood vessels, and that the inactivity of the brain arises from this condition. But it is not obvious what determines the contraction of these vessels at the definitely recurring period of sleep. It is probable that the nervous tissue of the brain is, as are the muscles of the body, poisoned or choked (as it were) by the chemical products of the day’s activity, and so readily cease to be active until the injurious products have had time to be carried away by the blood stream. Muscular substance undoubtedly is affected in this way, and that great muscle the heart, though never resting for a lengthened period, rests after each pulse or contraction, and recovers itself in the brief interval.

It is also probable that the exhaustion by the day’s activity of the oxygen stored up in the various tissues of the body produces a condition of quiescence whilst the store is replenished. Stimulation of the nerves through the sense-organs of sight, hearing, and touch will prevent or retard this natural quiescence, and the cessation of that stimulation is favoured first of all by the darkness of night and by the closing of the eyelid, as well as by the removal of clothes which more or less irritate the skin; also by the would-be sleeper taking up a position of perfect rest, and by the exercise of his will, withdrawing his brain as much as possible from all external influences. The would-be sleeper also controls, when possible, that internal stimulation of the brain which we call attention. It is the failure (owing to unhealthy conditions) to control the latter which leads to the most serious kind of sleeplessness, when the brain gets for hours out of restraint and works incessantly like an independent existence. The disturbance of the nervous system set up by irritation of the digestive organs, whether accompanied by pain or not, is an independent cause of sleeplessness which often co-operates with the first, and is (through the mechanism of the nerves) often set going (though it may arise independently) by an unhealthy excess in the excitement of the brain’s activity. There is no panacea for sleeplessness; the only thing to do is to consult a first-rate physician, and strictly follow his advice.

There are many irregularities and abnormal manifestations of sleep. There is the sleep which is induced by drugs such as opium, chloral, and alcohol, and that induced by chloroform, ether, and nitric gas. There is the heavy sleep accompanied by stertorous breathing, and there is the unconscious condition called “coma.” Then there is the prolonged sleeping called “trance,” of which that of the Sleeping Beauty, only to be broken by a kiss, is an example. It is not possible, in the present state of knowledge, to give an adequate account and explanation of the condition of the brain in these different forms of sleep, nor of the causes which induce that condition. One of the most interesting forms of sleep is the condition called “somnambulism,” or sleep-walking, in which part only of the brain is asleep, and other parts connected with various degrees of mental activity are in waking order. Sleep-walking is a condition which occurs spontaneously. On the other hand, “hypnotism” is the name for a peculiar kind of sleep produced intentionally by an operator on a patient by certain treatment and direction. In one of the stages of artificially induced hypnotic or “mesmeric” sleep—called the somnambulic stage—only so much of the brain is asleep as is concerned with conscious memory. The brain receives stimulation through the sense-organs, and the patient has the eyes open and appears to be awake. In this state he is peculiarly open to suggestion by words, which can be made to set up the most extraordinary illusions and consequent behaviour. On “waking” the patient has no memory of what has occurred, though a suggestion received in the somnambulic stage may persist in the unconscious memory, and cause conduct on the part of the patient (many hours after the brief hypnotic sleep has passed) which is entirely inexplicable by the patient himself or by those who are not aware of the fact that he had received a “suggestion” or “direction” when in the hypnotised state. The senses of smell, hearing, and touch are often abnormally acute in a hypnotised patient, but there is no evidence to show that the brain of such a person can be influenced or “communicated with” excepting through the ordinary channels of the sense-organs. “Day-dreaming” and “reverie” are conditions resembling the hypnotic sleep. The brain of each of us is constantly doing much of its work in a state of partial hypnotism, and the term “unconscious cerebration” has been used to describe it. A most interesting and difficult chapter of the study of mental disease belongs here.

The prolonged sleep of some animals in the winter, called “hibernation,” seems to be closely similar to ordinary sleep, but is set up by the depressing action of continuous cold instead of by the daily recurring quiescence of night and by the exhaustion due to the day’s activity. Many animals—such as the marmot and dormouse, the frog and the snail—exhibit this winter sleep. It has been found by experiment that even in midsummer the dormouse can be made to “hibernate,” by exposing it artificially to a low temperature, and hibernating animals can be roused from their long sleep by bringing them into warmth. During the winter sleep hibernating animals take no food, the pulse is slowed down, and the body temperature falls. The scattered fat of the body, and fatty matter and other material stored in special structures called “hibernating glands,” are oxidised and slowly consumed during this period, which may last for three or even four months. The animal on waking is often in a very emaciated condition.

It is undoubtedly the case that the human natives of high latitudes (such as the Norwegians), where there is no night in full summer, and where there is prolonged darkness in winter, have acquired the habit of keeping awake for many days in succession in summer, whilst making up for the loss of sleep by excessive indulgence in it during the winter. It is by no means clear how far man is capable of resisting the demand for recurrent daily sleep without injury to health. Undoubtedly many men are compelled by their avocations to sleep by day and wake by night. The length and duration of “spells of sleep” and the power to sleep little or not at all at one season, and almost uninterruptedly at another, without injury to health, are matters of habit, occupation, and circumstance. We have no ground for saying that every man “ought” to sleep eight hours or more per diem, or, on the contrary, for insisting that he should only sleep five or less. All depends on what he is doing when he is awake, and what other people are doing (so as to disturb him) when he is asleep; and we do not even know whether ten or twelve hours’ sleep would injure a man, were he able to take it, nor can we suggest how it would injure him supposing it did not interfere with his feeding and exercise.

As to quantities of sleep, there is the curious fact that the amount habitually taken in the civilised communities of this part of the world differs at different ages. Babies sleep a good part of the twenty-four hours, and probably schoolboys and schoolgirls (under our present conditions of life and work) ought to be given ten hours or more. Whilst adult men sleep from six to eight or nine hours, it is a curious fact that old people—not very old people, but those of sixty-five or thereabouts—often find themselves unable to sleep more than four hours at night, and take an hour or two in the daytime to make up for the deficiency. I remember hearing Mr. Darwin state this as to himself to his physician, Sir Andrew Clarke, who said it was very usual at his age, and difficult to explain, since at a greater age, when a man is called “very old,” a more or less continuous somnolent condition sets in. The father of a great judicial dignitary of these days, himself a barrister in large practice, when he was sixty years old would snatch fifteen or twenty minutes’ sleep at any and every opportunity throughout the day, even at the midday meal sometimes, so as altogether to disconcert those who were with him, and he told me that he never slept more than four hours at night, but got up and commenced work at four in the morning. The cessation in early old age of the desire for more than half the amount of sleep taken by younger men suggests that the regulating cause of the number of hours which are needed for sleep may be simply and directly the actual amount of work done by body and mind. This imperceptibly becomes less as men grow older, and so less recuperative sleep is necessary, though what work they do may be more effective and better adjusted to its purpose when they have arrived at the condition which is called “old age.”

We have seen that sleep in its widest sense comprises the simple condition of quiescence brought about in even the minutest living things by the recurring night, as well as the strangely elaborated varieties of cessation of activity in the whole or parts of the brain of man and of his body. Some of these cessations of activity naturally and spontaneously occur in unsophisticated mankind, when darkness falls on the earth at each succeeding evening. And it is hardly possible to doubt that a tendency to periodic sleep has become fixed in the substance of living things by the alternation of night and day—as well as in some cases by the change of the seasons.

I must conclude these notes about sleep by relating a very curious case of sleep, resembling the winter-sleep of higher animals, on the part of a snail. This was the case of a desert snail from Egypt, which was withdrawn into its shell, the mouth of the shell being closed with a glistening film secreted by the snail, as is usual with snails in this country in winter when they sleep. The desert snail in question was affixed to a tablet of wood in a glass case in the natural history department of the British Museum on March 25, 1846. On March 7, 1850, that is four years afterwards, it was noticed by a visitor looking at the case that the snail had emerged from his shell and discoloured the paper around, but had again retired. So the officials unlocked the case and removed the snail from the tablet and placed him in tepid water. He rapidly and completely recovered, crawled about as a wide-awake snail should, and sat for his portrait. This may be regarded as an instance of unusually long sleep, natural to this species of snail, and related probably to the frequently prolonged dryness of the snail’s surroundings.

We are led by such a case as this on to what are called examples of “suspended animation.” Wheel-animalcules, and some other minute creatures which are found living in tiny pools of water, on the bark of trees, and in the hollows of leaves, naturally dry up when the water evaporates. You may dry them yourself in a watchglass; they appear as nothing more than shapeless dust particles mixed with the dried mud of a drop of dirty water. They may be kept in this state for months—even years. I do not know that any limit has been ascertained. But when you add pure rain-water to the dust in the watchglass, it softens, and in less than an hour the little wheel-animalcules have softened too, and expanded into life, swimming about whilst the delicate spikes on their “wheels” vibrate regularly as though they had never ceased to do so, and as though the animalcules had not for years been dried-up little mummies.

Of course, the term “suspended animation” has been applied in earlier times to the often exaggerated stories of “trance” and deathlike sleep in human beings. But it is now with more justice applied to these instances of dried animalcules which return to life when wetted, and to similar cases of prolonged retention of vitality by seeds, since it would appear that in these dried animalcules life really is actually and totally suspended, although the mechanism is there which resumes its life when the necessary moisture is supplied. In cases of trance in man and hibernation in animals, the heart is still very slowly and feebly beating, and the breathing is still—almost imperceptibly—at work. The chemical changes are still very slowly and gently proceeding. The buried Indian wizard, and the snail, and the Sleeping Beauty are moist, and chemically active, though feebly so; life is not absolutely suspended. But in the dried animalcule (though complete chemical desiccation is not effected), the removal of the water from the body actually arrests the changes which we call life, just as a needle may arrest the balance-wheel of a watch. Supply the water, or remove the needle, and life ceases to be suspended; it goes on once more (as one of the rules of Bridge ambiguously enacts) “as though no mistake had been made.”