Because of this I think I should be excused for introducing the opinion of Aristotle among all the philosophers, that is, for choosing it from among them, for if we show it to be equally probable with the others which we presume to refute, it will be because, unlike them, it extinguishes them by its own plausibility.
Aristotle thinks sleep to be produced by a vapor generated by the heat energy in food, the fumes of which, rising to the brain, are there converted into moisture by the coldness of the brain itself, because, as he says, cold is felt at the contact just as rain is known to be made from vapor in contact with cold air; which moisture or humor, by force of gravity, is pushed downward, descends through the veins, drives the heat from the heart, and makes that cold also; whence, the cold spreading about, sleep, he says, generally arises.
This moisture, or humor,—ought to be warm, he writes; when it is cold, sleep will not be produced,—just as those affected with sleepiness show that their systems are in warm and at the same time humid condition; and children, who have abundance of this warm moisture, sleep the most; whence he states that this sleep chill has in fact its causes at the outset in this very warmth. These things he discusses partly in the book on “Sleep and Waking,” partly in the second of “The Parts of Animals,” and in “Problems.” I am not able to judge concerning the first matter, the idea of giving a single cause of sleep. For, according to this author, waking brings sleep;[12] since even animals, by means of waking and exercising their functions are known to become quiet and sleep, and it is said by him, that since animals become helpless in sleep, this helplessness is produced by the excess of waking that precedes it.
But not in waking, in food, or in this reason of generated vapor, is it possible to place the cause of sleep. Exercise produces this very effect. For through labor, deep and sweetest sleep falls upon the creatures; and this not on account of vapors rising from food, nor on account of a natural moisture, so much as by the violent exercise of the body. Foods that are cold or dried, as the hull of the mandrake,—taken into the body, induce sleep; which therefore is not accomplished because these foods give rise to vapors, since they would rather banish the vapors by their dryness; nor would these foods supply to the head what vapor is generated, since, repelled by the cold of such substances being taken into the body, the moisture would be repelled and chilled, and prevent the vapor from being carried to the brain. This would be so, as he says, only if the generation of vapors, and their ascent arose from the heating of this natural humor or moisture.
Other chapters of interest in Argenterio’s “Sleep and Waking” are:
Chapter II: What may bring sleep, and by what method, according to Galen.
Chapter III: The causes producing sleep, which are thought true.
Chapter XI: In what way sleep may be produced from natural heat.
Chapter XIII: Concerning natural causes of Sleep and Waking.