Popular Science Monthly/Volume 63/September 1903/Theories of Sleep
|THEORIES OF SLEEP.|
UNIVERSITY AND BELLEVUE HOSPITAL MEDICAL COLLEGE.
IT is not easy to define the condition of sleep in terms that will not admit of many exceptions. We readily recognize the states of rest and activity, but where the element of consciousness must be considered we are at once upon uncertain ground. If we think of sleep as an unconscious state, sharply contrasted with waking, we do well to limit our use of the word to the case of man and the most intelligent animals. Sleep in this sense is only to be associated with highly developed nervous systems and its final explanation is to be sought in events taking place in the brain.
Various writers upon the subject of sleep have turned their attention to quite different aspects of the matter. Some have undertaken to show why there is the need of sleep and why the tendency to sleep comes on at the close of each day. These writers have dealt with general or systemic causes. Others have concerned themselves with the cause of the unconscious—or dreaming—state in which the sleeper lies. They have endeavored to suggest intimate and local causes. Since the several theories are thus distinct in their application they are not necessarily mutually exclusive.
Broadly speaking we feel sure that the need of sleep follows from general or local fatigue. During waking hours the decomposition processes of the body doubtless rise above the life-long mean, and sooner or later there must be a compensatory fall below the average. The adaptation of the race to alternating light and darkness has made this rhythmic rise and fall to coincide with day and night—though less rigidly under the artificial conditions of civilized life than in more primitive times.
Fatigue at bottom is a chemical phenomenon, and so the theories of the first class are chemical. When a muscle has been stimulated until it exhibits the well-known signs of fatigue, there are two possible inferences—either this means an exhaustion of fuel substances or an accumulation of poisonous waste. Analogous views have been supported in regard to the chemical changes that lead to sleep. We have had an exhaustion theory advanced by Pflüger and an accumulation theory offered by Preyer.
Pflüger's theory has little experimental evidence in its favor. We know that a bloodless muscle may be subjected to a vacuum and made to part with its free oxygen, but that it is still capable of doing much work and of giving off carbon dioxide. In other words, oxidation may take place in the absence of free oxygen. Of course there is but one reasonable explanation, namely, that there is a store of the element in loose chemical combination. This store in the cells is spoken of as 'intra-molecular' oxygen, and its amount may be supposed to vary between rather wide limits. Pflüger pointed out that during the day, the katabolic processes being above the average, this hoard might be reduced until the lack of it should lead to the depression of functional activity and the suspension of consciousness characteristic of sleep.
Perhaps no one will maintain that this theory is adequate by itself. If there were nothing but intra-molecular oxygen to be considered, we should expect that a day of idleness would leave one fresh and bright at bed-time and that severe exercise for half a day might make a long sleep a pressing necessity. Pflüger's idea seems to explain more readily the sensation of being tired than that of being sleepy, which is so often quite independent of the other.
The alternative theory is to the effect that the waste-products of metabolism are not fully and promptly removed as they are formed during the day's activities, but gradually clog and poison the system until torpor is induced. The lactic acid produced in muscular contractions is held responsible for a great part of this toxic process. Acidity of the blood produces coma, and whatever reduces its normal alkalinity might be expected to favor sleep. Many objections to this theory suggest themselves. It does not explain why many people are at their best late in the day, nor why the onset of sleep is relatively sudden, nor why we are sleepy in the height of digestion when the blood is most alkaline. It is perhaps less easy to assail it if we suppose that the waste-products in question are not at large in the blood but have accumulated in certain cells, especially of the nervous system. In this case we need not assume a large quantity of these narcotic poisons, but only a peculiar distribution, and we can see why mental work is quite as fatiguing as physical work.
The transition from wakefulness to sleep seems rather abrupt, but is not instantaneous. Motor control is generally lost before sensation, and most people agree that of the avenues of communication with the world without, hearing is the last to be closed. This order of events is reversed in waking, when the alarm-clock or the unwelcome call is heard for an appreciable time before the eyes can be opened or a definite sense of one's situation realized. The sinking into sleep is favored by the removal of all that may excite either the attention or the reflexes. Darkness, quiet, bodily comfort and mental serenity are therefore sought. Sleep may be prevented by any of the contrary conditions—light, noise, pain or anxiety. When it is necessary to contend against drowsiness, one instinctively seeks objects for attention or sensory stimulation, such as may be secured by taking a slightly uncomfortable position. Evidently sleep presupposes a release of the brain from many stimuli and may be warded off by seeing to it that no such release is granted. There is on record the case of an unfortunate boy who had no cutaneous sensibility, was blind in one eye and deaf in one ear. His mentality was of a low order. To cause him to sleep it was only necessary to cover the serviceable eye and ear for a few moments. Here the waking condition was clearly dependent on an unceasing flow of sensory impulses into the brain. A person of higher intelligence, similarly afflicted, would doubtless sleep much less readily, for trains of thought might keep him awake in default of external stimuli.
The approach of sleep is accompanied by distinct vascular changes. The blood stream is shifting its bed. A most imperious summons to sleep comes from the dryness of the eyes, the sign, probably, of a lessened blood-flow through the tear glands. At the same time the temperature of the skin rises, possibly excepting that of the extremities. There is evidence then of a dilatation of the cutaneous vessels as sleep comes on, and the final passage into unconsciousness is accompanied by a considerable further dilatation. These vascular changes have been nicely gauged by what is known as the plethysmographic method, where the subject lay with one hand and forearm fixed in a glass cylinder filled with water. An increase of blood in the arm displaced water from the cylinder and a delicate recording apparatus showed how this dilatation came on with sleep and passed off with waking. An account of such experiments, of more than technical interest, is that contributed by Dr. W. H. Howell to The Journal of Experimental Medicine (Vol. II.).
It is generally inferred that the cutaneous dilatation at once reduces the general blood-pressure and the quantity of blood flowing through the brain, by diverting a large share to the skin. The lowering of pressure has been demonstrated by Brush and Fayerweather; the fact that there is anemia of the brain during sleep has been established by direct observation. An English physiologist. Hill, has been led to believe that the dilatation of blood-vessels that relieves the brain in sleep is not limited to the skin, but shared by the arteries of the digestive tract. That this is so is difficult to prove, but it is suggestive that a heavy meal is followed by a long sleep in the case of the lower animals and often with us by a hard struggle with drowsiness.
Cerebral anemia may be merely a concomitant of sleep, but it has frequently been held to be its immediate cause, the cells having previously been fatigued and suffered a lowering of functional capacity which has made them increasingly susceptible to depressing influences. This is the basis of Howell's theory. He has suggested that the exhaustion of the vaso-motor center is what induces sleep. We know that this center is in tonic activity, sending out impulses which hold the blood-vessels in a state of constriction greater than is natural for them. This tonic activity can only mean constant metabolism in the cells of the center. Furthermore the center is subject to the play of afferent impulses from all parts of the body. It is reflexly spurred to action by every sensory impression through eye or ear. It is called to respond in an appropriate manner to every change of posture or other muscular movement. It does not escape the effects of psychic processes, emotional states. Nothing is more natural than to suppose that the nerve cells of the center become fatigued by this unceasing activity. After the hours that we habitually number in a period of waking it responds less and less readily to the demands made upon it. It begins to lose its grip, so to speak, on the superficial and perhaps the splanchnic vessels. The blood supply to the brain tends to become less and the pressure in its arteries to be reduced. The subjective consequence is drowsiness. If it is resisted by fixing the attention or by exercise, the center rallies temporarily under the spurring, contracting the vessels and turning the tide of blood back into the brain. But the anemia soon returns, and the drowsiness becomes more compelling. When the person lies down, a flood of sensory impulses that have been pouring in from the contracting muscles is suddenly checked. The eyes are closed and the stream of visual impulses ceases. With the withdrawal of this reflex stimulation and the acquiescence of the will the center relaxes still further its hold upon the cutaneous vessels, the blood-flow in the brain becomes more reduced, and unconsciousness comes on. During sleep the vaso-motor center is responsive to stimuli from without as the plethysmographic experiments show. A sufficient stimulus produces waking, and seems to operate by turning back into the brain a sufficient quantity of blood displaced from the contracting vessels of the skin. Such a stimulus must be a strong one in the first hour or two of sleep, but later a much weaker one will answer. Several physiologists have tested the depth of sleep at different hours of the night, judging of it by the height from which a weight must be dropped that the sound of its fall shall arouse the sleeper. All have agreed that the greatest depth of sleep is reached as early as the second hour. According to one writer it becomes steadily more shallow from that time until the end. Others have observed a second, minor deepening toward morning. Many people will agree that their own sensations seem to imply such a second period of comparatively profound sleep.
What we call natural waking in the morning is usually due to some stimulus from without—light or sound—which would not have roused one from the deep sleep of midnight. But the stimulus may come from within, as from the state of certain organs or, curiously enough, from the previous resolution to wake at a certain time, which often operates with something of the compulsion of a hypnotic suggestion. Howell supposes that during sleep the nerve-cells of the vaso-motor center are gradually restored to prime condition and hour by hour become more irritable. So it is easier and easier as time passes to induce the vascular changes that involve waking. Moreover, the recuperated center resumes something of its normal tonic activity before consciousness returns, and so the final step is taken with none of that sense of violence that accompanies a sudden waking from sound sleep. The border-line is likely to be crossed and recrossed several times before the waking state is well established. When one is fairly roused mental activity and the pouring in of sensory impulses keep him from further napping.
Now what peculiar condition can be conceived to exist in the brain during the period of anemia and unconsciousness? What microscopical changes may be supposed to mark the transition from wakefulness to sleep? Oddly enough, the two hypotheses which are extant are quite opposite in character. The first, which has attracted the greater notice, is that of Duval. He has suggested that consciousness depends on the contact of cell-processes in the brain whereby effects are propagated from neurone to neurone. If sensory impulses are to alter consciousness, there must be a pathway for their passage. If a single synapse on the course of such a pathway is rendered impassable, the message from the sense-organ is lost from conscious life. If every sensory path is interrupted at any point between the periphery and the cortex, there must be insensibility as to the outside world and the state of the body. If all motor paths are likewise broken, there can be no voluntary action. If, in the third place, the association paths are also severed, there can be no synthetical processes of thought, no ideation. In short, the brain must lose its individuality by the breaking of connections between its structural elements. If we could suppose that every synapse in the central nervous system might be snapped, and impassable gaps opened between the cells wherever one had been wont to influence another, there must be an end of consciousness, for, in utter isolation, these cells could no longer combine their activities into one whole such as forms the physical basis of psychic life. A much more local disruption of connection, limited perhaps to the cortex, might be sufficient to explain the subjective condition in sleep. At any rate, Duval's view is that the cortical cells are capable of retracting or extending their processes so as to sever and resume their relation with neighboring elements. Experimental evidence in support of this theory is naturally slight. Wiedersheim has described amœboid movements on the part of cells in the nervous system of a small transparent crab. Of course it is only in such lower forms that the living cells can readily be brought under the microscope. Duval himself suddenly beheaded dogs that were awake and others in anæsthesia and made histological preparations from the brains. He believed he could distinguish the sleeping brain by the more contracted and isolated appearance of its cells.
The second histological theory of sleep, which has been said to be quite opposed to the first, is that of the Italian neurologist, Lugaro. Both demand the capability of amœboid movement on the part of the cells. But while Duval supposes that in sleep the cells have broken their contacts, Lugaro supposes that they have made new contacts with great freedom. At first thought this view seems unreasonable. A multiplicity of contacts and added pathways in the brain might be supposed to imply a richer and keener consciousness. But this would be true only to a certain point. When the indiscriminate combination had gone a step further mental confusion might be expected, then fantastic associations and a meaningless mosaic of memories—practically a state of dreaming. Let the cells commingle their impulses still more freely and consciousness will be lost, for the diffusion of energy in the brain will result in a lessened intensity of flow in the principal channels. If each cell scatters its communications in every possible direction no definite effect in consciousness is to be looked for. According to Duval, the cells which are affected in sleep can not discharge; according to Lugaro, they may do so, but the resulting impulses are utterly dissipated in a maze of by-ways. Waking, according to Duval, is the resumption of intercourse among these cells; according to Lugaro, it is the restriction of intercourse to habitual and purposeful channels.
There is no reason why we may not be eclectic in regard to these two points of view. It may be that many paths are interrupted in sleep, while others are opened. In the hypnotic state it is clear that many paths are blocked, including those by which the will of the subject habitually asserts itself, while others, especially those making connections between the auditory and motor areas, transmit impulses with extraordinary efficiency. This condition is explicable if we suppose that certain synapses are broken, as Duval imagines, and that the tide of nervous impulses pours with intensified energy through the narrowed outlets remaining—an idea borrowed from Lugaro. If we consider that a man is most thoroughly awake when his attention is most rigidly concentrated, when he is a 'man of one idea' we shall perhaps incline toward Lugaro 's conception of sleep, which is certainly as far as possible removed from this mental fixedness. Hypnosis is accompanied by cerebral congestion and natural sleep by anemia. There is accordingly a strong temptation to suppose that the cell-changes in the two states are opposite in their nature, that in hypnosis the retraction of the dendrites is characteristic and in natural sleep their extension. The sluggish condition of the mind under suggestion as compared with its fanciful flights in dreaming falls happily in line with this view. But such speculation is premature.
It was said at the outset that the several theories of sleep are not all mutually exclusive. It is possible to go beyond this statement, for we may assign a place to each of those mentioned without inconsistency. We may suppose in the first place that the alternation of day and night through the ages has impressed its rhythm upon the race, so that it is hard for the individual to break from the habitual course in which activity is associated with light and rest with darkness. In other words, the amount of the metabolism tends to keep above a mean for some hours and then to fall below it. The excess of destructive processes over those which are recuperative during the waking hours results in general and local fatigue, a condition into which may enter both the depletion of intra-molecular oxygen and the accumulation of toxic waste-products. While this progressive loss of condition affects the body as a whole, the nervous system is subject to its own peculiar drains. It is very probably the hard-worked vasomotor center which proves to be the vulnerable spot. With its release of the blood-vessels in certain areas from its reenforeing influence comes the cerebral anemia. Then, we may suppose, the nerve-cells become less active than in the brain which has its full supply of blood, that they cease to send impulses over the usual routes, either because gaps have opened or because such impulses as do arise are permitted to stray and be scattered, producing no effect in consciousness or one which is quite bizarre and meaningless.
Such an outline as this is a composite scheme in which the conditions emphasized by Pflüger and Preyer are given recognition as fundamental causes of sleep, Howell's idea is accepted as explaining well its onset, its varying depth and the awakening, while the pictures sketched by Duval and Lugaro arc combined to represent the intimate state of the slumbering brain.