Sleep is symptomatic of repose in the brain and nervous system; in those who are awake cerebral activity is unceasing. Potential energy is being constantly expended in mental operations, vital processes, muscular movements, &c.; also in the perception of impressions that stream towards the brain incessantly from every part of the economy and of the environment. This is attended by wear and tear of the nervous textures, and by the deposition in them of waste-products proportionate to the work effected. Waking is therefore a positive condition—one in which energy is consumed more quickly than it is restored, and than the waste-products are eliminated. It is a state that entails a full working blood-supply to the brain. After a time, longer or shorter, a sense of fatigue suggestive of repose supervenes that is only relieved by sleep. Sleep, on the contrary, is a negative state—one in which these processes are reversed. The brain is inactive; consciousness and volition are in abeyance; coincidentally the central blood-supply is diminished, the brain is smaller in size, and its temperature is lowered. Expenditure of energy is curtailed to the greatest extent compatible with life. Recuperative processes continue, and predominate over the destructive; the nervous structures are depurated, and potential energy is accumulated. When recuperation is completed awaking occurs, and is accompanied by feelings of invigoration. Waking and sleep, therefore, are indicative of cerebral activity and of cerebral rest; they are both physiological functions of a healthy nervous system. When either is unduly prolonged, curtailed, or modified it gives evidence of a departure from health.
The brain is a composite organ. It contains innumerable centres which dominate an equally vast variety and number of functions; and these require and obtain sleep in varying degrees according to their functions. The supreme (psychical) centres which are constantly alert during waking need more sleep than some of the subordinate centres. These latter rest partially by day, so that the complete abdication of their functions at night is less urgently required.
The incursion of sleep is gradual. It is synchronous with the subsidence of activity in the brain and nervous system, and it is best studied in this connection. Molecular activity does not cease in the whole brain instantly, but in one portion after another until the whole organ becomes tranquil. It is suspended first in the centres situated highest in the cerebrum, and afterwards in those at lower levels, till the medulla oblongata and spinal cord are included. It consequently affects the motor centres of the brain first. The earliest symptom of sleep is the weakening of the voluntary muscles; relaxation takes place in one set after another until the body assumes the horizontal posture. The sphincter muscles are exceptional; they remain contracted. During deep sleep true slackening occurs in the muscular fibres, even in diseases like tetanus and hydrophobia. A man may fall asleep in motion, as in walking or riding, and maintain his equilibrium; a certain amount of activity being sustained in the motor centres enables him to do so. If he yields to sleep in a stationary attitude, the motor centres being quiescent, he tends to sink into the recumbent posture. Activity next wanes in the psychical centres of the brain. The will ceases to control the working of the intellectual faculties, and the perceptive powers are lessened. The mind, no longer inhibited from within nor corrected from without, revels in absurdities until mental operations cease, or, at least, till they are wrapped in oblivion. The centres of the special senses in the brain are next involved, and usually in the following order; they fail to perceive slight or ordinary impressions, while their special nerves transmit them in a slow and imperfect manner. (a) Vision: The eyelids close; the eyeballs turn upwards and inwards in the orbits. The eyeballs occasionally, if rarely, move independently, and not in unison, during sleep. The pupils contract—the contraction being in ratio to the depth of sleep; they dilate widely in the act of awaking. (b) Hearing: Loud noises interrupt sleep, though an expected noise does so more readily. Monotonous sounds do not prevent or interrupt sleep; their cessation may terminate it. (c) Smell: Slight odours may prevent the onset of sleep; but very pronounced smells are required to disturb it. (d) Taste: It is difficult to determine the condition of this sense during sleep, the sense of touch being apt to vitiate the conclusion. (e) Touch: This is the most sensitive sense during sleep. It is chiefly through it that man is warned of danger, and that his safety is secured. Reflex acts can always be elicited, their vigour depending on the strength of the stimulus, the degree of sensitiveness of the part to which it is applied, and the depth of sleep. The centres in the medulla oblongata then become less active. They chiefly dominate the heart, the lungs, and arterial pressure. The heart beats from ten to twenty times less per minute, and the blood pressure is diminished. The respirations are about four fewer per minute. The inspirations are shallower and chiefly thoracic. The pause between inspiration and expiration is practically absent. Pettenkofer and Voit stated that of the total carbonic acid eliminated in twenty-four hours 58 per cent. is given off during the twelve hours of day and 42 per cent. during the twelve hours of night; whilst 67 per cent. of the oxygen taken in is absorbed during the twelve hours of night and 33 per cent. during the day. The centres that dominate the various secretions only respond to suitable stimuli, and as these are wanting during sleep the secretory organs are less active. The digestive juices are not formed, and if the gastro-intestinal movements do not cease entirely, they are greatly lessened. The urine is secreted in one-fourth the quantity. The quantity of sodium chloride, sulphates, and urea it contains is smaller; the latter is decreased one-half. The secretion of sweat is increased, and that of milk is continued.
In consequence mainly of the inactivity of the bodily functions the bodily temperature falls as much as from 0°·5 to 2° F., chiefly for some hours after midnight; it being lowest about 4 A.M. It falls similarly by day in those who work by night and sleep by day. According to Helmholtz, a man gives off about three times more heat when he is awake than he does when he is asleep. During sleep the economy is more vulnerable to the influence of deleterious surroundings; it has less resisting power. Hence it is that infectious and malarial diseases are so readily contracted by sleeping in places in which these poisons abound; hence also the liability to catch cold from sleeping in a draught. Briefly summarised, during sleep the brain, ganglia, medulla oblongata, and spinal cord are in a state of repose. And the extent to which they are involved may be inferred from the fact that sleeping persons suffer little, if any, from shock in accidents which affect those who are awake most injuriously.
Sleep varies in depth in persons of different ages. It is usually profound in the young, and light in the aged. The depth bears some relation to its duration. Natural sleep varies from time to time during the same night. The observations of Kohl-schitter, of Rummo and Ferrannini, and of Mönninghoff and Piesbergen showed that it was deepest an hour after its onset, when its intensity decreases quickly, then more slowly, and again it becomes more profound a second time after four to five hours. This is important in sleep-disturbance, for excitations will act more powerfully when it is lightest. On the other hand, when it is deepest the lowest centres are least inhibited, and the blood is less thoroughly oxygenated. It is then that epileptic, convulsive, spasmodic, and such-like seizures are most apt to occur. Cases of sleep lasting for weeks and months have been recorded (that of Johann Latus in the hospital of Myslowitz in Silesia in 1891-92 lasted months); in Britain it occurs in neurotic patients, and sometimes is called trance. It is pathological and not natural sleep. In western Africa 'sleeping sickness' is a well-known fatal disease.
Cause of Sleep.—For upwards of two thousand years continuous attempts have been made to elucidate the cause of sleep without success; many theories have been promulgated, but they have fallen short of explaining it.
Of circulatory theories the one that prevailed during many centuries attributed sleep to a determination of blood to the head; and much evidence was adduced in its support. Nevertheless, congestion of the brain is incompatible in health with good and refreshing sleep. This hypothesis was replaced by the anæmic theory. From the observations of Donders (1854), Durham (1860), Regnard (1868), Hammond and Weir-Mitchell (1869), and Ehrmann, Salathé, and F. Franck (1877), it would seem to be conclusively demonstrated that the blood-supply of the brain is lessened during sleep. Whether this is causative, concomitant, or consequent is still open to conjecture. Pathological anæmia is a well-recognised source of wakefulness. It appears certain that a blood-supply sufficient for nutritive purposes, but insufficient for mental activity, is an essential factor in natural sleep. The brain, like all other organs, governs and is not governed by its vascular supply.
Humboldt suggested one of several chemical theories—that sleep was due to the want of oxygen; and many have since urged that everything that deprives the brain of oxygen conduces to sleep. An atmosphere deficient in oxygen, or one containing an excess of carbonic acid, induces drowsiness and sleep, but it is poisoned, and not healthy sleep. Pflügger believed that activity of the psychical cells depends on the quantity of intra-molecular oxygen they contain, and that all function is attended (particularly in the gray matter) by explosive movements in the cells, caused by the combination of oxygen with the tissue-elements, in which oxygen is consumed and carbonic acid is formed. He thought that these movements used up the oxygen at a greater speed than the circulation could replace, and that a time arrived when the lack of oxygen and an excess of carbonic acid caused drowsiness and sleep. Again, it has been urged that the waste-products (resulting from mental operations) in the nervous textures occasioned sleep. Preyer believed that the accumulation of these (ponogenes) at first caused fatigue, then sleep, and that waking occurred when they were eliminated in sleep. This doubtless is a factor in the causation of sleep, but it does not explain it.
The absence of external stimuli, such as occurs in the darkness and quietness of night, has been credited with causing sleep. It is a desirable aid to sleep, but, in health, sleep ensues despite all disturbing agencies—even on the torture-rack. Sleep has been ascribed to the law of periodicity that governs all organised things. The study of the bodily functions shows that rest and activity alternate—even the heart gets about ten to twelve hours rest in the pauses between its contractions. Of other numerous theories, it will suffice to say that no one of them can be accepted as fully explaining and finally settling the subtle problem. It may be that the mystery will never be solved until a deeper insight into the working of the mind itself is acquired.
How long should one sleep? is a question that must be answered generally. No hard and fast law can be laid down; every man must be a rule to himself. It is quite clear that nature intends everyone to sleep until the effects of waking are dissipated, and until the bodily energy is renovated. In childhood, when the constructive processes of growth involve large expenditure of energy, sleep is long and profound. In youth much sleep is still needed. In middle age, when decay and repair alone require to be balanced, less sleep is required. In old age, when repair is slowly and imperfectly effected, more sleep is desirable. The duration of sleep is also largely influenced by sex, temperament, occupation, habit of sleep, season, climate, &c. The true guide is the recuperation of the energies; that is indicated by a feeling of well-being on awakening. Some persons awake, after sleeping for an apparently sufficient number of hours, worn, jaded, and exhausted. That is an indication of disease, pointing to an inability on the part of their textures to accomplish their reparation.
Sleeplessness.—The effects of curtailed sleep are very marked. The loss of a single night's sleep is attended by symptoms of enervation; that of several nights' sleep, by serious consequences. In many diseases the lack of sleep determines a fatal issue; conversely, sleep often determines recovery. Few persons appreciate the fact that the voluntary curtailment of sleep for an hour every night for a year is equivalent to the loss of forty-four nights' sleep of eight hours' duration. That represents an enormous expenditure of energy—e.g. during that time the cardiac pulsations are approximately 210,000 and the respirations about 90,000 more frequent than they would have been during sleep. Undue curtailment of sleep is attended by loss of flesh, the essence of emaciation being the preponderance of decay over repair. The skin becomes dry, and it loses its suppleness and translucency. All the bodily textures are badly nourished, and the organs suffer from lack of innervation. When sleep has been limited unduly, inability to sleep ensues and becomes the bane of existence. Exhaustion of the cerebral structures leads to still graver symptoms, such as insanity. Indeed, insomnia precludes or enters to a greater or less extent into the causation of almost every form of mental alienation. They arise from similar causes. Many of the tragedies of life are due solely to the prolonged want of sleep. Insomnia or continued sleeplessness may cause death from exhaustion. Sleep, on the other hand, is a powerful curative agent—one that alone suffices to determine recovery. It has been well called 'the chiefest thing in physic.'
Sleeplessness is a symptom designed to subserve a highly conservative purpose. It attracts attention in some to ailments that might otherwise elude detection; in others it maintains consciousness requisite for the preservation of life. Sleeplessness is symptomatic of cerebral activity. This may be originated and perpetuated in divers ways. It may arise from mental causes, for the mind is dependent on the brain for its external manifestation. In mental overwork, worry, grief, suspense, &c. the intellectual or emotional centres respectively grow hyperæmic. At night the vaso-motor nerves that dominate the arterial vessels, worn out by continued inhibition, fail to moderate the supply of blood to these centres, and so the man is impelled to work and to worry, and he is prevented from sleeping. In the early stages he is long in falling asleep, and when he does it is to dream of his work and worry. Eventually insomnia becomes pronounced. Sleeplessness may depend on faulty conditions in the brain itself. Cerebral cells primarily or congenitally feeble, or secondarily debilitated by unhealthy conditions, display their enfeeblement in an incompetency to accumulate nervous force. Their molecules are rearranged in a preternaturally unstable manner, and they discharge their latent energy in response to trifling excitations. Such cells are designated 'explosive.' The peripheral cells generally participate in the enfeeblement, and they and their conducting nerves transmit impressions in a rapid and exaggerated manner. The reaction-time is lessened. This is a fruitful source of wakefulness in neurasthenia and its allied conditions. Sleeplessness from this cause usually appears at the epochs of life, and under privations or strains. The patient gets to sleep easily, but awakes in two or three hours, to remain awake till five or six in the morning. The awakening is often attended by distress and misery. It is in this class of cases that insomnia often occurs and recurs mysteriously. All neurotic diseases tend to recur paroxysmally; a bad sleep-habit once acquired is liable to return from time to time from very trivial causes.
Sleeplessness may result from a blood-supply altered either as regards quantity or quality. The cerebral cells are very sensitive to their environment; they bear excess and deprivation badly. Hyperæmia and anaemia of the brain are consequently productive of cerebral unrest and of wakefulness; and these brain-conditions may arise from diseases of the heart, lungs, and other organs, which secondarily implicate the cerebral circulation. The brain textures are unfavourably affected by alterations of vascular tension; high tension is unfavourable to sleep. They are equally disturbed by overheated blood such as pertains to feverish states. They are similarly perturbed by blood contaminated by peccant matters, such as those of gout, rheumatism, syphilis, malaria, &c.; hence the sleep disturbance in these diseases, and also that which follows the excessive use of tobacco, alcohol, &c. In the latter class the patients get to sleep quickly, to awake at two or three in the morning, to lie awake for some hours; and they are often awakened by slight external excitations.
Sleeplessness may be originated by sensory stimuli, which cause contraction of the peripheral and dilatation of the cerebral blood-vessels, acceleration of the heart's action, and an increase of arterial tension. They also cause dilatation of the pupils, and change the character of the respirations; these become deeper, quicker, and more abdominal. Consciousness of external things can alone be aroused by impressions transmitted to the brain textures (themselves insensitive) becoming converted into sensations and ideas through the agency of the mind. When excitations are of sufficient intensity to cause sensations wakefulness is inevitable. Excitations may be instigated in the most varied ways; they include pain of all kinds, and many impressions arising out of digestive and other derangements. They affect the neurasthenic, gouty, and rheumatic unduly.
It is evident that successful treatment depends on accuracy of diagnosis. In each and every case the sleeping conditions of the sufferer should be attended to. Exercise and food should be proportionate to the body and to the strength of digestion. The præmie via and the state of the skin must not be neglected. A good habit of sleep should be assiduously cultivated. In sleeplessness due to mental causes change of work should be advised, that new centres may be called into operation, and the overwrought ones correspondingly relieved. It is in these cases alone that devices for getting to sleep, such as the monotonous counting of figures, sheep, &c., are efficacious. In bad cases the judicious use of pure hypnotics is helpful; they force the mind into oblivion. Drugs, like potassium bromide and chloral hydrate, which act chiefly on the brain centres that function thought, and induce sleep that closely resembles natural sleep, are called pure hypnotics, in contradistinction to many substances, as narcotics, analgesics, anodynes, soporifics, &c., that arrest pain, diminish sensibility, and disturb in a pronounced manner the functions of the brain and nervous system.
When wakefulness is due to neurasthenic conditions, tonics, increased nutrition, the abandonment of excesses, change of air and scene are indicated; electricity and suitable baths are useful adjuncts; hypnotics should be sparingly employed. Sleeplessness due to hyperæmic conditions of the brain is to be met by antiphlogistic treatment; that which depends on anaemia, by tonics, &c. In both, posture during sleep will repay attention. That which originates from gouty, rheumatic, syphilitic, and like poisons only yields to the alleviation of the constitutional state.
In sleeplessness due to sensory stimuli and pain, care must be taken to ensure the removal or mitigation of these, that the sleep they prevent may supervene. Medicines—analgesics or anodynes—such as belladonna, should be first selected, for they act as indirect hypnotics by operating chiefly on the nerve-terminals. When such remedies fail, narcotics, as morphine and opium, are called for; they not only lessen perceptivity in the brain and sensitivity in the nerve-endings, but they diminish the conductivity of the nerves. Such wide-reaching drugs have many disadvantages in such cases, but pure hypnotics are useless. Hypnotics, narcotics, and sleep-inducing medicines should never be taken except under medical advice. To drug the brain into quiescence without remedying the cause of the cerebral activity will in many cases only aggravate the evil, for they will be complicated by a drug-taking habit. In most instances drugs are best avoided.
Good bibliographies of this subject are to be found in Copeland's Diet. of Med. (vol. iii. pt. 2, Lond. 1858); in Diet. Encycl. des Sciences Médicales, art. 'Sommeil' (ser. iii. tom. 10, 1881); also in Neale's Medical Digest (Lond. 1891). The following may be consulted in connection with the physiology of sleep: Cappie, The Intra- cranial Circulation (Edin. 1890); Carpenter, Mental Physiology (Lond. 1874); Durhan, Physiology of Sleep (Guy's Hosp. Reports, 1860); Fleming, 'On Sleep,' Bril. and For. Med. Chir. Rev. (1865); Hammond, Wakefulness, &c. (Phila. 1865); Holland, Mental Philos. (Lond. 1858); Lemoine, Du Sommeil au Point de Vue Phys. et Psychol. (Paris, 1855); Lyman, Insomnia (Chicago, 1885); Maryand, Le Sommeil et l'Insomnie (Paris, 1881); Moore, On Going to Sleep (Lond. 1868); Mosso, Sulla Circolaz. del Sangue nel Cervello, &c. (Rome, 1880); Pfügger, Theorie des Schlafes (Pfügger's Archiv, 1875); Serguéeff, Physiol. de la Veille et du Sommeil (Paris, 1890); Vulpian, Leçons sur l'Appareil vaso-moteur (Physiol. et Pathol.) faites à la Facul. de Méd. de Paris (tom. ii.). In reference to sleeplessness the following may be referred to: Bride, Sommeil et l'Insomnie des Vieillards (Lyons, 1888); Fouquet, L'Insomnie (Montp. 1867); Hammond, Sleep and its Derangements (Phila. 1869); Macfarlane, Insomnia and its Therapeutics (Lond. 1890); De Mussy, Sur l'Insomnie (Paris, 1866); Pierrot, L'Insomnie (Strasb. 1869); Sawyer, Causes and Cure of Insomnia (Lond. 1886), and many other papers in medical journals. See also the articles ANESTHESIA, CATALEPSY, COMA, DREAMING, HYPNOTISM, INSANITY, NARCOTICS, SOMNAMBULISM.
In Plants, sleep is one of the phenomena of Irritability (q.v.). Light acts on plants as a powerful stimulus, essential to their active and healthful vegetation. When it is withdrawn the flowers of many plants close, and the greater number show a tendency to it, whilst leaves more or less decidedly incline to fold themselves up. The leaf-stalk also generally hangs down more or less, although in some plants it is more erect during sleep. The sleep of plants, however, is not always nocturnal. The flowers of some open and close at particular hours of the day. Thus, the crocus is a morning flower, and closes soon after mid-day; whilst some flowers expand only in the evening or during the night. It was Linnæus who first observed the sleep of plants in watching the progress of some plants of lotus, the seeds of which he had sown. The periodic movements of plants, of which sleep is one, have various uses. The opening of flowers in the morning and the closing in the evening is related to pollination; the folded petals protect the stamens and other sensitive parts from excessive cooling or wetting. The rising and falling of foliage leaves is useful in the same way, and preserves the chlorophyll-bearing tissue from injury; the extended position is adapted for temperatures and intensities of light that are favourable to assimilative activity. See PLANTS (MOVEMENTS OF), and VEGETABLE PHYSIOLOGY.