SLEEP, that state of the body in which, though the vital functions continue, the senses are not affected by the ordinary impressions of external objects.
Under the article DREAMS, the nature of sleep has been investigated so fully, that little remains to be said in this place, besides the obviating a few objections that may be made to the account of it there given.—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
Sleep. 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. The greatest difficulties that occur in giving a solution of the phenomenon of sleep is, that it may be brought on by seemingly opposite causes. Thus, though moderate fatigue will induce a propensity to sleep, the same may be brought on by eating a full meal without any fatigue at all. In like manner, tho' moderate fatigue will induce sleep, too great a degree of fatigue will prevent it altogether.—This, however, may easily be solved, on the principles laid down in the abovementioned article. It is there supposed that the mass of blood contains the vital principle, or what is called the animal spirits, like a general reservoir, to be distributed through the whole body. Part of the blood is sent to the brain, where another collection or reservoir is made, serving in a particular manner for the purposes of reason, memory, and the rest of our mental faculties. The rest of the blood is distributed through the whole body, where some part of the vital spirit is taken up by each of the small extremities of the nerves, and by means of the nerve communicates a certain sensation to the brain when these extremities are affected by any external object.—As long as the blood continues in a sound state, and can supply a certain quantity of these spirits, the person remains awake; but when the supply is interrupted, by whatever means, he necessarily falls asleep. This is the case with a person when moderately fatigued; the blood being then deprived in some measure of part of that animating principle which is necessary to the exercise of our external senses, or for keeping us awake. If the fatigue be carried to too great a length, the efficacy of the blood itself is injured, and it becomes incapable of retaining the necessary quantity for carrying on the operations of life; which, though independent of our senses, are maintained by means of the same principle with them. Hence, though the person remains awake, he is far from being in health; and if the watchfulness continues for any considerable time, he is in great danger of becoming delirious.—After a person has taken a full meal, he is seized with drowsiness, because the fresh chyle continually pouring into the subclavian vein, requires the exertion of great part of the vital power to assimilate it to the blood, so that enough is not left to affect the senses so readily as usual. In like manner may we understand the sleep which follows the immoderate use of spirituous liquors, of large doses of opium, &c. These seem to act by rarefying the blood to such a degree, that the fine vessels are overstretched, and compress the small nervous fibres which every where accompany them, so as to destroy for a time their power of action. In the same manner do the vapours of charcoal or fixed air, anyhow applied, seem to act. Hence the best cure for persons affected by these things is to cool them as much as possible; and hence also we may see the reason why drunk people will bear an exposure to much greater degrees of cold, without injury, than such as are sober.—How cold itself induces sleep, is easily understood. The whole body is then so benumbed, that neither the blood can part with its animating principle, nor can the nerves take it up even if it should
do so; and hence the fatal sleep which ensues in such cases.
From what has been advanced concerning the nature of sleep, we may easily see, that its uses are to restore the spirits and strengthen the body, though too great indulgence in this respect has a contrary effect. The night is the most proper time for sleep, by reason of the absence of the solar light, which animates the whole creation, and in some measure prevents that waste of spirits which is the consequence of watching for any length of time. Hence we see why a person who has sat up all night finds a revival of his spirits in the morning; and why nocturnal watchings are prejudicial to health, even though a person should indulge himself in sleeping throughout the day.
Some animals require much more sleep than others, and many continue even for months in this state. For the human species six or seven hours are generally sufficient; though from habit many require eight, nine, or even ten hours. Those who addict themselves to study, in general require much sleep, on account of the great waste of spirits which intense thought occasions.