On Sleep and Waking

​Indeed, for this reason they are also voracious eaters, despite being hard in body; for their bodies are disposed as if they have derived no benefit from anything. And black bile, being cold by nature, makes both the nutritive region and the other parts cold, wherever such a residue may potentially exist. Thus it is clear from what has been said that sleep is a certain concentration of heat inwards and a natural antiperistasis due to the aforementioned cause. Wherefore the movement of the sleeping person is great. And from wherever it departs, that part is cooled, and because of the cooling the eyelids droop. And the upper and outer parts are cooled, but the inner and lower parts are hot, such as the regions around the feet and the interior parts. Yet someone might raise a difficulty, that sleep becomes strongest after food, and that wine and other things possessing such heat are soporific. And it is not reasonable for sleep to be a cooling, while the causes of sleeping are hot. Does this then happen because, just as the stomach when empty is hot, but its filling cools it on account of movement, so also the passages and regions in the head are cooled as the exhalation is carried upwards? Or, just as with those who have hot water poured over them a sudden shivering occurs, does the cold concentrated there also, when the heat ascends, cause cooling, and make the natural heat grow weak and retreat? Furthermore, when a large amount of food is introduced, which the heat carries up, is the heat cooled, just like fire when logs are piled on, until the food is digested? For sleep occurs, as has been said, by the bodily matter being carried up by the heat through the veins to the head. But when it is no longer able to sustain it, but the matter carried up exceeds in quantity, it is pushed back again and flows downwards. Wherefore, as the heat that carries it up is withdrawn from below, humans fall down (for they alone of animals are upright), and when the matter falls upon them it produces unconsciousness, and later an image. Or, while the explanations now mentioned are indeed possible causes for the cooling to occur, nevertheless the region around the brain is sovereign, as has been said in other works. For the brain is the coldest of all parts in the body, and for those animals that do not possess it, the part analogous to it. Just as, then, the moisture evaporated by the sun’s heat, when it reaches the upper region, is cooled due to its coldness, and having condensed is carried down, becoming water again; so too, in the ascent of heat to the brain, the residual exhalation condenses into phlegm (which is why catarrhs also appear to originate from the head), while the nutritious and non-disease-causing exhalation is condensed and carried down, cooling the heat. And to being cooled and not receiving the exhalation easily, both the thinness and the narrowness of the veins around the brain contribute. This, then, is the cause of the cooling, even though the exhalation is excessive in its heat. And waking occurs when the food is digested, and the compressed heat, being large in quantity in a small space, overcomes the surrounding cold, and both the more bodily blood and the purest blood are separated. Now the thinnest and purest blood is that in the head, while the thickest and most turbid is that in the lower parts. And the source of all blood, as has been said both here and elsewhere, is the heart. And of the two chambers in the heart, the middle one is common to both; and each of those two receives from each of the two veins, namely, the so-called great vein and the aorta; and in the middle chamber the separation takes place. But to define these matters is more appropriate for other discussions. And because the blood becomes more unseparated after the taking of food, sleep occurs, until the purer part of the blood is separated into the upper parts, and the more turbid into the lower parts; and when this happens, they wake up, having been released from the heaviness arising from food. Therefore, what the cause of sleeping is has been stated, namely, the antiperistasis in a mass upon the primary sensory organ by the bodily matter carried up by the connate heat; and what sleep is has been stated, namely, the fettering of the primary sensory organ so that it cannot function, occurring of necessity (for it is not possible for an animal to exist without the occurrences that bring it about taking place), but for the sake of preservation; for rest preserves it.