SLEEP. This term is employed to designate that state of suspension of the sensory and motor functions which appear to alternate in all animals with the active condition of those functions, and which may be made to give place to it by the agency of appro priate impressions upon the sensory nerves. This definition, which we have borrowed from Dr. Carpenter's article on " Sleep" in Todd's Cyclopedia of Anatomy and Physiology, may seem somewhat complex, but cannot be simplified without rendering it less stringent. The necessity for sleep arises from the fact that the exercise of the animal functions is in itself destructive of the tissues of the organs which minister to them, so that if the waste produced by their action were not duly repaired, they would speedily become unfit for further use; and it is on the nutritive regeneration of the tissues which takes place during true healthy sleep that its refreshing power depends. While the sensory and motor functions are suspended during the condition we designate as sleep, the organic functions are uninterruptedly carried on, the respiratory, cardiac, and peris taltic movements proceeding with equal uniformity during the sleeping and waking states.
There can be no doubt that the state of sleep is one to which there is a periodical tendency, and that this disposition is so arranged as to correspond in its recurrence with the diurnal revolution of the earth. Although in man and most animals night is, from its darkness and silence, the natural period for repose, yet there are numerous exceptions to the rule. For example, amongst lepidopterous insects, butterflies are active during the day, hawk-moths during the twilight, and moths during the night. Amongst birds, the goat-sucker, or night-jar, and the owls, are nocturnal, and, as a general rule, the same is the case with carnivorous animals. The causes of sleep may be divided into the direct and the predisposing. The direct cause of sleep is that feeling of exhaustion or fatigue which is usually experienced when the waking activity has continued during a consider able portion of the twenty-four hours—a feeling that the brain requires repose; and, in fact, unless the brain lie in an abnormal condition, sleep will at last supervene, from the absolute inability of that organ to sustain any further demands upon its energy. Among the predisposing causes which favor the access of sleep, we must especially notice "the absence of sensorial impressions; thus darkness and silence usually promote repose; and the cessation of the sense of muscular effort which usually takes place when we assume a position that is sustained without it, is no less conducive to slumber."—Carpenter's Human Physiology, 6th ed. 1804, p. 592. On the other hand, persons accustomed to live where there is a continuous noise, as in the neighborhood of mills or forges, often can not sleep if the noise is suspended. These cases, however, probably fall within the next general predisposing cause—namely, the monotonous repetition of sensorial impressions. Thus the droning voice of an unimpressive reader or preacher, the gentle ripple of the ocean, the hum of bees, the rustling of foliage, and similar monotonous impressions on the auditory nerves, are usually provocative of sleep. In these and similar cases the influence of the impressions is exerted in withdrawing the mind from the consciousness of its own operations, and in suspending the directing power of the will; and this is the case, says Dr. Carpenter, "even when the attention is, in the first instance, volun tarily directed to them, as in some of the plans which have been recommended for the induction of sleep, when there exists no spontaneous disposition to it. In other methods the attention is fixed upon some internal train of thought, which, when once set going, may be carried on automatically, such as counting numbers, or repeating a Greek verb.
In either case, when the sensorial consciousness has been once steadily fixed, the monot ony of the impression (whether received from the organ of sense or from the cerebrum) tends to retain it there; so that the will abandons, as it were, all control over the opera tions of the mind, and allows it to yield itself up to the soporific influence. This last method is peculiarly effectual when the restlessness is dependent upon some mental agitation, provided that the will has power to withdraw the thoughts from the excit ing subject, and to reduce them to the tranquilizing state of a mere mechanical repeti tion The access of sleep is sometimes quite sudden, the individual passing at once from a state of complete mental activity to one of entire torpor. More generally, however, it is gradual, the mind while remaining poised, as it were, between sleep and the oppo site condition being "pervaded by a strange confusion which almost amounts to wild delirium; the ideas dissolve their connection with it one by one; and its own essence becomes so vague and diluted that it melts away in the nothingness of slumber."—Mac nish, Philosophy of Sleep, p. 21. The amount of sleep required by man is affected by so many conditions (among which must be especially mentioned age, temperament, habits, and previous exhaustion), that no general rule can be laid down on the subject. The condition of the fetus maybe regarded as one of continuous slumber: on its first entrance into the world, the infant passes most of its time in sleep, and this is particularly the case in children prematurely born, such children seeming only to awake for the purpose of receiving food. During the whole period of growth, in which it is necessary that the constructive operations of the body should preponderate over the destructive processes, an excess of sleep is required; and by the time that adult age has been attained, and the onstruetive and destructive processes balance eath other, the necessary amount of sleep lies gradually fallen to about one-third or less of the diurnal cycle. In very old age, again, in consequence of the deficient energy of the nutritive process, a larger amount of sleep is required. With regard to the influence of temperature, it is observed that a plethoric habit of body usually predisposes to sleep, while thin wiry people of a nervous temperament require comparatively little sleep. Persons of lymphatic usually great sleepers, but this is probably due, as Dr. Carpenter suggests, to the fact, that " through the dullness of their perceptions they are less easily kept awake by sen sorial or mental excitement" than persons of a happier temperament. The influence of habit is by no means inconsiderable on the amount of sleep required by individuals, and this influence may be brought to act on the protraction as well as the abbreviation of the usual period: as extreme examples, we may mention that gen. Elliott, celebrated for his defense of Gibraltar, did not sleep more than four hours out of the twenty-four (which is probably the smallest allowance for rest compatible with a life of vigorous exertion); while Dr. Reid the metaphysician, could take as much food, and afterward as much sleep, as were sufficient for two days. Moreover, the influence of habit in producing an aptitude for repose, or a readiness to wake at particular periods, is well known. The sleep of soldiers during a siege, of sailors or others who must take their rest as they best can, will often conic on at command; nothing more being nec essary to induce it than to assume a recumbent, or, at all events, an easy position, and to close the eyes. Thus, capt. Barclay, in his celebrated match, in which he walked 1000 m. in 1000 successive hours, very soon got into the habit of falling asleep the moment he lay down.