There is a peculiar form of sleep that is styled incubus, or nightmare, where a great degree of pain and uneasiness exists, but where sleep still continues, and this in opposi tion to our efforts to dispel it. We appear to have a kind of imperfect consciousness of our state, and to be aware that, it' we could execute our volition we should remove the uneasiness, yet this we are unable, for some time, to accomplish. The accession of incubus is supposed to be generally attended with some local pain or uneasiness, frequently arising from an affection of the stomach or the organs of respiration; not unfrequently it is to be regard ed as a morbid affection, or at least as a symptom of dis ease in some of the organic functions.
There is another kind of dreaming, which is still more peculiar, where the sleep, so far as respects the effect of external impressions, is unusually profound, but where the voluntary power over the muscles is only partially sus pended; it is styled somnambulism, or sleep-walking. Al though it requires a great degree of external violence to rouse the individual from sleep, yet he is able to perform many of his ordinary occupations; he can move about the house, utter sentences, and can even write, or play upon an instrument of music. To perform these actions, it is necessary that the organs of sense should be in exercise ; yet it appears that they are almost, if not altogether, in sensible to other impressions. Where, for example, a person in this state is engaged in writing, the sight must be directed to the paper and to the writing implements, yet the eye has been found to be insensible to all other surrounding objects. In one respect somnambulism essen tially differs from common dreaming, that whereas, in this latter state, the ideas appear to pass from one circum stance to another, in the most unconnected manlier; in the former, they are firmly directed to one object. Al though somnambulism, when it exists in a slight degree, seems often to arise from some casual or incidental cir cumstances, yet, when more confirmed, it has appeared to originate in, or to be connected with, a morbid condition of the nervous system, and even to terminate in derange ment of the mental faculties.
The second subject which we proposed for inquiry, the efficient cause of sleep, is one which is involved in much obscurity. There are many circumstances which lead us to conclude that sleep is an affection of the nervous sys tem, and that the state of muscular action is only so far affected by sleep, as that we lose our voluntary power over the muscles. A favourite hypothesis with the earlier physiologists was, that sleep depends upon an exhaustion, as it was termed, of the nervous power ; that this power, existing in a limited quantity only, and being expended by the ordinary occupations of life, a certain period of time is required for it to accumulate, in order that it may per form its appropriate functions, There is no doubt a por tion of truth in this hypothesis; but it must be acknow ledged, that it is rather a simple expression of a general fact than an explanation of it. It gives us no light re specting the nervous power, or the mode of its produc tion; it does not inform us whether the state of sleep, in any way, favours this production, or whether it merely prevents its expenditure. Some of the most eminent mo
dern physiologists, as hailer and Hartley, conceive, that the physical cause of sleep consists in an accumulation of blood in the vessels of the brain thus pressing upon the organ, and impeding its functions; an hypothesis which was supposed to be proved by the effect of morbid collec tions of fluid within the skull, in producing a state of lethargy or coma. But, upon this opinion, we may remark, that the state produced by pressure upon the brain only resembles sleep in the circumstance of the partial or total abolition of the nervous faculties, while it differs from it essentially in many of its other phenomena ; at the same time that it is highly improbable that a daily occurrence, and one of the most salutary operation, should be identical with a morbid affection of a very dangerous nature, and which is generally followed by an irreparable injury to the functions of the part. We may, upon the whole, con elude, that we have no certain knowledge respecting the efficient cause of sleep, and that we are not acquainted with the physical change that takes place in the nervous system.
In a medical and pathological point of view, it often be comes a matter of great importance to ascertain what are the circumstances which tend to promote sleep ; and, ac cording to the view of the subject which has been taken above, we may arrange them under two heads; 1st, Those which consist in removing every circumstance which may excite the nervous system into action ; and, 2d, Those which render it less sensible to the impression of ordinary stimuli. With respect to the first head, we may remark, that every function requires a certain portion of stimula ting power in order to produce its action; for, without this stimulus, neither sensation nor motion would take place. If we, therefore, carefully abstract all external im pressions, and as much as possible prevent the mind from dwelling on its own ideas, sleep generally ensues, unless it be counteracted by some morbid cause. Among the other causes of sleep, those which act by diminishing the sensibility of the nervous system, we may place, as the most effectual, a moderate degree of fatigue, both mental and corporeal, by which the nervous power is expended, and thus less easily excited. Various narcotic drugs ap pear to have the power of directly acting upon the nervous system, and diminishing its sensibility ; and if it be not carried to too great an extent, the same effect appears to ensue by preventing the blood from experiencing its clue change during its passage through the lungs. hence we may conclude that the most favourable combination of circumstances for the promotion of sleep, are moderate fatigue, absence of pain, light, noise, and other circum stances calculated to produce a strong impression upon the nerves or organs of sense, and, above all, a tranquil state of mind. This last is, indeed, probably the most im portant of all the circumstances that hare been enume rated, for we find that persons in a rude state of society, and young children, immediately fall asleep when the body is at rest; but that, on the contrary, while the mind con tinues in agitation, no attention to external circumstances will induce sleep.