Question Two
On bodily sleep, comprising three questions
Little Question One of Question Two
What bodily sleep is and whence it is caused
It is asked what bodily sleep is and whence it is caused.
To this it is answered that, according to the philosophers, sleep is the withdrawal inward of the vital and animal spirits, one of which is the vehicle of heat and the other the vehicle of a sensible species.
Or, as others say, sleep is a fixed suspension of the animal, that is, the sensitive, powers from their acts.
In the common course this occurs because the diffusion of spirits, or of another influence necessary for sensations, is impeded from the heart or brain. For as long as this influx is impeded, the sensitive powers cease and rest from their acts.
The said influx can be impeded, or fail to occur, for many causes and in many modes.
First, as Albert says, it can be impeded on the part of the body, as when a smoky evaporation ascending through the veins from the digestive members reaches the cold region of the brain. There the smoke, condensed by that cold and descending, constricts the nerves and obstructs the other passages of the spirits to the organs of the exterior senses. Thus deprived of the spirits required for their actualization, those organs do not perceive exterior objects.
Others state the same thing more clearly in this way. The said influx is impeded by smoke ascending from the digestive members to the heart, or to the place in the head—that is, the brain—where the nerves which convey the spirits converge, or where the natural spirits are sublimated into animal spirits.
That smoke can impede the said influx either by filling the nerves, by compressing them, or by constricting them when they have been reduced to coldness. Sleep can also come to be naturally on the part of the body without such smoke, because of some stoppage or another kind of obstruction of the interior passages.
Sleep is also caused on the part of the soul through a concurrence of heat and spirits toward the place of some injury which torments one vehemently within.
Sometimes it is caused by a defect of heat and spirits in their root. For when the spirits fail, it is necessary that the senses cease from their acts, as happens in the insane when instances of insanity occur from depletion, that is, weakness.
Sometimes it also occurs through a certain withdrawal or recalling of the spirits to the interior because of some interior passion, according to Luke 22 and Matthew 26: “Their eyes were heavy, and they slept because of sadness”; or because of another accident, as in syncope.
Sometimes sleep also occurs from vehement fatigue, attenuation, or continual contemplation, because of which things the interior powers require assistance, as is evident in rapture and ecstasy.
Sleep is also sometimes produced from outside, that is, from the sweetness of some melody through which the exterior senses fall away from their acts. On account of this, as the Philosopher says in Book VIII of the Politics, melodies were performed in temples by which some persons were rapt.
Sometimes sleep is also produced from the vehemence of sudden terror, and sometimes from an injury outwardly inflicted upon the head or another member, as when a human being falls upon his head or draws out blood.
These things are from Albert the Great and Lord Henry of Hesse, On Genesis.