Popular Science Monthly/Volume 46/December 1894/The Chemistry of Sleep

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WE do not comprehend the mystery of wakeful consciousness; and therefore that of temporary unconsciousness, including sleep of all kinds, is equally beyond our understanding. To say that sleep is a suspension of our control over our thoughts and our motor nerves and voluntary muscles is a mere substitution of other classes of mysteries equally inscrutable.

The same should be frankly admitted as to all our so-called explanations of natural phenomena, which consist mainly of generalizations, expressing in few words the laws that rule and connect, so far as we can discover, the infinity of cosmical facts and transformations. Such generalizations—whose surpassing importance and value are nevertheless undeniable—imply, as essential preliminaries, the laborious classification of the facts in any field of investigation that we have in hand.

But in the field of which we are now to attempt a brief and necessarily superficial survey it must be conceded that such classification is as yet highly imperfect. The forms and modifications of existence varying in cause or origin, nature, and degree which may be called by the general name of sleep have not yet been subjected to the exact and critical experimental research needed for scientific classification. Little can now be done except to point out wherein our knowledge is defective, and to indicate some more or less tentative arrangement of the facts under different heads as a provisional guide to the study of this condition of existence, in which mos persons expend one third of their terms of life. Such heads may be as follows:

An attempt at a definition of normal healthy slumber, which only is entitled to be called—

"Tired Nature's sweet restorer, balmy sleep."

Such a definition must be sufficiently well founded on the conditions presented in natural sleep to admit of drawing lines of parallelism with and of divergence from other species of lethargy or unconsciousness, or modified consciousness, that arise from abnormal or morbid conditions, mental and nervous disorders, drugs, anæsthetics, cold, heat, exhaustion, partial asphyxiation by drowning or otherwise, etc.

A statement of our present narrow range of facts and observations relative to the chemical, physical, and physiological changes of the organs of the body, and of their functions, during normal sleep.

A consideration of the mental, moral, and emotional phenomena presented in natural and in artificial sleep, including its morbid forms, with those forms in which the mind sleeps with the body awake, and those in which the body may sleep with the mind more or less awake.

Anæsthesia in general, inclusive of its widely differing successive stages, as that of exaltation of some mental faculties, with or without full consciousness, or one of the waking dream conditions (see further on—) that of extinction of full consciousness with the muscular system still awake—and, finally, that of complete lethargy of the mind and body with extinction of all sensation.

An attempt, which must needs yet be quite crude, to generalize something useful from the sparse and scattered array of facts thus found to be available.


As to a definition of natural sleep, it may be interesting to go back a century and examine the views then held. Having at hand the second edition of the Encyclopædia Britannica, published from 1778 to 1783, the writer finds therein an article on Sleep, in the tenth volume, which article is brief, being but supplementary to that on Dreams, in the fourth volume. To this latter subject we shall return further on. Under Dreams, the following meager definition of sleep is given: "Sleep is a state in which all communication is cut off between our sentient principle and this visible world." By this, taken literally, a blind man would be asleep. But, of course, the word visible was intended to imply the whole world of the senses. Still, allowing such latitude, the definition is both inaccurate and inadequate. As to the asserted complete cutting off of external impressions from the senses in true sleep: were this so, a sleeping person could not be awakened by the usual means—namely, a forcible external impression upon one or more of the senses.

Its inadequacy may be briefly illustrated by pointing out that it takes no note of one of the most salient phenomena of sleep—that the will, though not at all suspended therein, being easily recognized in dreams, altogether loses its rationality and its control over the workings of the mind as well as over those of the body.

The following may be set forth as an attempt at a reasonable and comprehensive definition, or rather description, of the conditions we find in sleep: Sleep is a state in which the impressions of external objects on the senses are dulled, but not annulled or suspended; in which the emotions, the imagination, the memory, and the will are but partially or even not at all suspended, and may even be intensified, while the control of the will over the emotions, the imagination, and the memory is wholly annulled, together with its conscious control over the nerves of the voluntary muscular system. Reasoning power is annulled, but involuntary and instinctive muscular motions, and those arising from habit, still continue. Memory is certainly not annulled, or we should never dream of the past, of which we do dream largely.

The consciousness of Duration—that deepest of all mysteries—is not absent in sleep, but also passes wholly out of the control of the reason and the memory, and loses all relation to the conditions of waking experience, being often exaggerated or exalted far beyond these. Unconsciousness of duration occurs during the waking dream state, being here suspended at times, so that lapses of time remain unnoticed. Those subject to intense mental occupation, or to reverie, which is a species of waking dream, will attest this. Strong emotions also influence our consciousness and perception of duration.

It may be objected that the above description of sleep does not cover a species that general readers have been taught to consider typical—what is called "dreamless sleep." This, however, is probably a literary fiction. Even the "dreamless sleep of infancy" is not always realized, for healthy infants often present indications of dreaming. Sleep so deep as to be dreamless is probably not of the most natural kind, but rather the result of nervous, mental, or muscular exhaustion, or of drugs, being thus a more or less morbid lethargy.

No one would claim that natural dreams are symptomatic of morbid conditions, though of course certain kinds of dreams—as those from opium, of delirium tremens, dyspeptic nightmares, etc.—are such. These, moreover, are often a kind of waking dream, in which the body remains more or less awake while the mind is lethargic or deliriously excited.

It thus appears that sleep is far from being the simple affair it was held to be in the last century. On the contrary, it is a highly complex phenomenon, involving all the functions—mental, moral, and physical, and doubtless also physiological—of that most complex of all organisms, man. Hence, it must be worthy of far closer scientific study than it has yet received.

As to the physiological and chemical phenomena that accompany sleep in the animal economy: Of this—the most essential branch of the investigation it must be stated that its cultivation has been very imperfect and inadequate. Still, enough is known to convince the present generation of chemical thinkers that sleep is by no means a subject for the psychologist alone, but will turn out to be physiological and chemical—that is, accompanied, and even almost altogether ruled, by chemical agencies and transformations. We have here a strong and sound basis for the hope of arriving at a control of natural sleep itself and at modes of influencing and promoting its production without danger or detriment to general health. The few facts we have so far may be stated as follows: In healthful sleep the blood circulates more slowly; hence kidney action and perspiration diminish. The breathing is slower, and the exhalations of moisture and carbon dioxide from the lungs are less. Hence, these are doubtless produced in smaller amount, as they should be, from the diminution of muscular work and of combustion of animal fiber. And it is highly probable, though not yet fully tested by experiment, that certain gaseous, vaporous, or other products of the transformation of the muscles, nerves, and other tissues, partially stored up in the blood during working hours, are eliminated during sleep by the lungs, skin, and kidneys. The digestion is stated to be more active during normal sleep, and the temperature in the vital organs at least is stated by some authorities to remain at its normal point, though in the limbs it probably falls. This may be due to a tendency of the blood to the internal digestive organs. It is known, as was proved independently by Hammond and Durham, that the volume of the brain diminishes during natural sleep.

Returning to the old edition of the Encyclopædia Britannica (1779) before quoted, a few sentences possessing interest may be further taken from the article on Sleep. Referring back to the article on Dreams, it is here added: "Sleep we have shown to arise immediately from the communication between our sentient principle and external objects being cut off, in consequence of which memory is also lost, and the person becomes insensible of existence. This state may be induced either by such causes as affect the brain, the nervous system, or the blood, though it probably depends in most cases on the state of the vital fluid." This "vital fluid" or "vital spirit," as it is elsewhere denominated, is apparently not the blood itself, but an agent or influence then unknown, believed to reside therein. If we were now—with our present chemical knowledge—to substitute for "vital fluid" oxygen of oxyhæmoglobin of the arterial blood, this part of the citation would become rational. Oxygen, however, had then been discovered but four years previously, and its functions must have been yet but vaguely understood. The Italics have been introduced into the above excerpt to emphasize certain notions about sleep whose fallacy has been previously pointed out.

Dr. Hammond suggests that in the most profound natural slumber the spinal cord, with its nervous ganglia, remains awake, though not quite so much so possibly as when the brain itself is wholly conscious. Of course, the involuntary muscles, as those which operate the heart, the respiratory organs, etc., governed by ganglia of the spinal cord, never slumber. Their slumber is the last sleep of all. But Dr. Hammond cites proof independent of this. The position of the sleeping person is often changed unconsciously when uneasy, and the feet, when chilled, will be drawn under the bedclothes. He mentions still more impressive facts, such as sleeping on horseback, without danger of losing seat, once the habit is acquired. We may add that men sleep leaning against a wall or a tree, or even astride of a limb, without serious risk of falling. Monkeys, opossums, and other animals sleep hung up by helices formed of their tails. Somnambulism may be referred to the same class of facts.

Prof. Michael Foster, of the University of Cambridge, England, a very eminent physiologist, says, in a work published in 1891, that although the phenomena of sleep are largely confined to the central nervous system, and especially to the hemispheres of the brain, the whole body shares in the condition. "The pulse and breathing are slower. The intake of oxygen and the output of carbon dioxide, especially the latter, are lessened. Indeed, the whole metabolism" (by which term he designates generally the chemical transformations in the living organism) "and the dependent temperature of the body are lowered." (The latter statement is not in accord with other authors.) "We can not say as yet how far these are the indirect results of the condition of the nervous system, or how far they indicate a partial slumbering of the several tissues." Prof. Foster distinctly declines to enter into what he calls "the psychology of sleep and dreams." But to ignore dreams and their essential relations to natural sleep suggests to the present writer the play of Hamlet with the Prince of Denmark left out.

Foster points out the fact, as the result of all experience, that the recumbent posture is highly promotive of natural sleep, and connects it with the reduction of energy consumed in the circulation, especially with the large diminution of work that falls upon the heart. The most remarkable and significant statements made by this high authority are that "during sleep the pupil is constricted, during deep sleep exceedingly so, and dilatation, often unaccompanied by any visible movements of the limbs or body, takes place when any sensitive surface is stimulated; and on awaking, the pupils also dilate." He adds that this "contraction of the pupils is worthy of notice, since it shows that the condition of sleep is not merely the simple and direct result of the falling away of afferent" (external) "impulses. When the eyes are closed in slumber, the pupils ought, since the retina is then quiescent, to dilate; that they are constricted, the more so the deeper the sleep, shows that important actions in the brain are taking place." In other words, as we have already set forth, certain inherent functions of the brain, also of the nervous system, emotional and imaginative processes, among others, independent of what he calls "afferent" impulses from without, are still fulfilled. The nerves which govern the muscles of the iris must be so affected by the imagination (or by some unknown agent acting only during sleep) as to contract, as if light still fell upon them. This does not seem, however, to concur with the generally accepted view that the muscular fibers that control the iris are involuntary. Until these extraordinary phenomena are understood, we shall doubtless not arrive at a complete theory of sleep. The study of dream phenomena will in this connection (in spite of Prof. Foster) constitute an essential aid.

The collateral phenomena accompanying and related to sleep, though not confined to natural sleep—or, for that matter, to any kind of sleep that suspends the bodily functions—are several. Dreams are the most important and significant; but we can give space only to some partial consideration of the two species of dreams namely, sleeping and waking dreams. Our encyclopædia of 1778 discusses several views then entertained regarding dreams; among them the curious one presented by the famous Richard Baxter, in his book on the Immateriality of the Human Soul, to the effect that dreams are due "to the action of some immaterial beings upon our souls." Objection is made to this on the ground that the existence of such immaterial beings is incapable of proof. This spiritualistic side of the discussion we must omit.

The most important suggestion regarding dreams made by this encyclopedist of 1778 is that mania and hypochondriasis are conditions under which the subjects dream while otherwise wide awake. That this is so appears incontrovertible, and we learn thence that the dream state does not necessarily arise from sleep, of the body at least. Mania may probably be defined as a dreamy slumber of the mind, without the participation of the body. Similar conditions are abundantly brought on temporarily by the abuse of certain powerful agents that destroy the rule of the will and reason over the mental functions, leaving the individual to be swayed altogether by the animal emotions. When the mania is chronic, accounts agree that in time the demoralization becomes, in most cases, so complete as to really give color to the views in ancient books, including our scriptural writings (and, moreover, in the works of some modern writers), that many if not most maniacs are under demoniacal possession. Waking dreams and reveries are known also to attack many persons in whose minds imagination is a predominating element. And even such as do not belong to this class, may become, when under the influence of great distress, fear, privation, or hardship, subject to such waking visions. Starvation or thirst, or both, have often produced the most remarkable illusions. One interesting example may be cited which is historical. It is found in the Journal of Strain's Exploration of the Isthmus of Darien in 1854,[1]many of the members of which perished slowly and horribly from sheer hunger (thirst not being a factor in this case). The following paragraph is given as an illustration:

"From the time that food became scarce to the close, and just in proportion as famine increased, they did not gloat over visions of homely fare, but reveled in gorgeous dinners. So strangely and strongly did this whim get possession of their minds, that the hour of halting, when they could indulge undisturbed in these rich reveries, became an object of the deepest interest. While, hewing their way through the jungles, and wearied and over-come, they were ready to sink, they would cheer each other up by saying, 'Never mind, when we go into camp we'll have a splendid supper,' meaning, of course, the imaginary one they designed to enjoy. Truxton and Maury would pass hours in spreading tables loaded with every luxury they had ever seen or heard of. Over this imaginary feast they would gloat with the pleasure of a gourmand, apparently never perceiving the incongruity of the thing. They would talk this over while within hearing of the moans of the men, and on one occasion discussed the propriety of giving up in future all stimulating drink, as they had been informed it weakened the appetite. As hereafter they designed, if they ever got out, to devote themselves entirely and exclusively for the rest of their lives to eating, they soberly concluded that it would be wrong to do anything to lessen its pleasures or amount."

"Absence of mind," of the results of which such remarkable stories are current, must be cited also as an example of a transient waking dream or species of reverie state.

Under the next head a few words may first be expended upon the class of lethargies or comatose sleeps arising from certain diseases, such as apolexies, epilepsy, paralysis, catalepsy, etc. These have but remote analogies with natural sleep. In most of them the brain, instead of being in the collapsed condition, partly drained of blood, which occurs in real sleep, is overloaded and congested with blood, and the resulting stupor is mostly absolute and dreamless. Syncope or fainting has one point of analogy with sleep—namely, that it is accompanied by cephalic anæmia, or loss of blood from the brain; but this goes so much further than in sleep, and the other symptoms, of decrease of general circulation and of respiration, are so great, that there is no further analogy. The insensibility of every sort during syncope is often absolute. Paralysis may in some forms be regarded as a like slumber of the body while the mind remains awake. The sleep from the action of excessive cold, which precedes death, is due, of course, to general retardation of the circulation, which in due time affects the brain, but probably has but a partial analogy with natural sleep. It has not been sufficiently examined to determine whether dreams accompany it at any stage. The delusions from the abuse of opiates and those of delirium tremens are of the class of waking dreams. Death by drowning is asserted to be preceded by extraordinary waking dreams, sometimes rather pleasurable than otherwise.

The most important of all species of sleep for our present purposes of comparison are those produced by anæsthetic agents. The general use of these is now but about half a century old, though it has been shown that the principle of anæsthesia and its use in surgery, etc., is about as old as the Christian era. Dioscorides describes the use of the mandragora root, steeped in wine, as an anaæthetic drink to produce a sleep in which painless amputations and other surgical operations could be effected, Pliny and Apuleius (author of the famous Golden Ass) each make similar statements. Sir B. W. Richardson announced, in 1875, that he had experimented with mandragora, and had largely confirmed the ancient stories about it. The moderns have a great number of anæsthetic agents, some of which are permanent gases, others volatile liquids, which are mostly administered by inhalation; but we yet know very little, in exact scientific detail, about most of them. Some, when given in properly graduated doses, furnish us with conditions approaching closely to those of natural sleep, generally of a dreamless kind; but others, if given in small doses, furnish us examples of sleep of the will power and reasoning functions, accompanied by emotional dreams and hallucinations. In the case of nitrous oxide (laughing gas), especially when given in moderate doses, mixed with air ox oxygen, emotional excitements of the most remarkable kind result. The subject will often fly into an energetic rage, manifesting a violent and ludicrous pugnacity toward those about him. Others will declaim in an oratorical manner, sometimes sensibly and coherently. Of such occurrences the subject often retains vague memories, like those of natural dreams. Thus memory persists during the action of this agent, as in true sleep; and, as in sleep, the will-control is wanting. The natural instincts, no longer restrained, rise up and assert themselves. "In vino Veritas." Ordinary alcoholic intoxication, indeed, is but one kind of anæsthesia, and passes in its progress through the same characteristic stages or phases that have been previously defined.

The generalization remaining to be brought forward is as follows: It has already been indicated that of all the known artificial forms of lethargy, the premonitory stages of anæsthesia only are approximate in character to normal sleep. Nitrous oxide, with oxygen, is now largely administered to the point of insensibility, by distinguished physicians, to produce vicarious sleep, in cases of obstinate insomnia, and is regarded as a priceless remedy. But the lethargy thus produced is dreamless, and therefore not normal sleep. The mental and moral natures of the individual are in a condition of suspended animation, which, from what has been previously set forth, is an abnormally torpid condition of life. Further, it has been shown that complete anæsthesia itself differs essentially from natural sleep; for in the latter the senses are only dulled, and still remain more or less susceptible to strong external impressions, by dint of which wakefulness is readily brought about.

The question now arises, Is there any agent which exerts in small doses a sedative effect on both body and mind, an effect that increases with the dose or with the period of action, until entire insensibility finally results, and which, like nitrous oxide, works no permanent harm? The answer is that we have one such agent, carbon dioxide gas; itself a constant and copious product of normal animal life.

It is placed by Richardson, an eminent specialist on this subject, among anæsthetics, and this place is generally conceded to it. Even in the day of Pliny, its outward effects were familiarly known, as at the Grotta del Cane, near Naples. Popularly it is deemed a deadly poison, but many chemists have held that it is not so. In mines and deep wells it is usually unmixed with air, and the result of inhalation is, of course, speedy suffocation. Berzelius long ago stated that air containing five per cent can be breathed without serious injury. Dr. Angus Smith found, nevertheless, that one twenty-fifth of this proportion produced a slow diminution of the circulation, such as we have seen accompanies natural sleep. We have one experiment made by a commission on coal mines of the British Association, in which an animal exposed to carbon dioxide and air, half and half, a small jet of air being, however, continuously introduced to maintain a supply of free oxygen, lived "for a long time"—how long not stated. Here, plainly, rapid poisonous action is shown to be absent.[2]

We have seen that during natural sleep the carbon dioxide expired (during wakefulness between three and four volumes per hundred of air from the lungs) diminishes as the circulation diminishes. Many claim that a succession of rapid but longdrawn respirations will quickly bring on drowsiness, and often sleep even ensues. This can be attributed only to a small overcharging of the circulation with carbonic acid. It must be recalled also that carbonic-acid waters and effervescent drinks generally produce with many a decided though transient sedative effect. But in this way doubtless none of the gas reaches the blood, except the little thrown off from the stomach by eructation, this being incidentally and partially inhaled. The gases contained in arterial and venous blood appear to vary somewhat. Several authorities agree, however, that the amount of free carbonic acid in arterial is much smaller than in venous blood, while the oxygen in the former (combined mainly with the hæmoglobin) is much larger. Pflüger made the free carbon dioxide, mostly in solution, in red arterial blood of a dog thirtyfour volumes and a half per hundred, while in the blue venous blood it rose as high as fifty, or half the volume of the blood.

The generalization referred to is simply that normal sleep and sleepiness, or drowsiness, are due to a small increase over the average of the carbonic acid in solution in the blood, arising through its overproduction from the greater amount of muscular and other tissue that undergoes oxidation during the waking hours. During the sleeping hours this overload of the anæsthetic gas is gradually discharged until wakefulness results.

In this brief discussion no room has been occupied with what are called hypnotism, clairvoyance, trance, mind-reading, etc. These are outside of our scope—being, if authentic, not natural but supernatural phenomena, pertaining to the realms not of law but of miracle.


Experiments, continued through many years, by Dr. S. Rideal, show that the chemical activity of sunlight during winter on the high Alps is much greater than at lower levels, and enormously greater than in large towns at the same season. This increased activity may contribute importantly to the beneficial effects of health of residence in such regions.
  1. A great part of this journal may be found in Harper's Magazine in 1881 (or there-abouts).
  2. New facts have appeared since the above was written that add great strength to the positions taken here against the generally assumed highly toxical qualities of carbon dioxide—a body always present in the lungs and blood of men and animals. The experiments of Berzelius above cited—like all others of that unparalleled chemical genius—as to the toleration of animal life for important amounts of this supposed poison, stand now as being much more than verified. To wit, among others, an English experimenter, Mr. T. H. Wilson, has found that rabbits are uninjured by breathing for an hour air containing twenty-five per cent of the much-dreaded gas. At the last meeting of the British Association, Dr. J. S. Haldane discussed the subject of the fatality to miners of "after-damp," hitherto believed to owe its effects to carbon dioxide. He shows that absence of oxygen is the real cause of death in these cases. He introduces the novel and highly rational suggestion that all parts of mines should be provided with reservoirs of highly compressed oxygen. A pint of this, he shows, would keep a man alive for an hour. Doubtless, with our present means and knowledge, oxygen could be stored in mines and elsewhere, in its liquefied form, for this and other valuable uses.—H. W.