Otsar ha-ḥayim (Treasury of Life)

On the preservation of one’s health when going to sleep and upon awakening. Our master, of blessed memory, wrote the following in laws 4 and 5 [Maimonides, Mishneh Torah, Sefer ha-mada, hilkhot de‘ot 4.4]:

Commentary

Being awake for overly long periods is very harmful to a person, for it destroys the spirits, especially the spirits [pneumata] of the brain, in the same manner as those of the whole body. In particular, it dries up the brain itself, increases the yellow bile, greatly heats up the blood, and causes fever and heat-related illnesses. Upon the subsequent loss of heat, one can develop cold-related illnesses. Therefore, due to the many agitations to which people are prone and the movement of all their limbs and senses, nature gave them sleep, through which they can rest from their strains and toil. Man receives numerous benefits from sleep: his strength for work is invigorated; the spirits, which dissipated through all his labor, are renewed; and his warmth is concentrated inwards. Furthermore, by means of sleep one’s food is well-digested, and this leads to the production of much good blood, and as a consequence of this upsurge in blood, one’s heat increases, and the entire body is revitalized. One’s anger calms, the soul is at rest, and excessive diarrhea is prevented. Sleep is very good for the elderly, as it moistens their dry bodies and they are relieved of their pains.

However, prolonged sleep is very harmful, as it renders one’s spirits and mind lazy, and it gathers together all the superfluities, and then one’s natural heat becomes like fire that is not blown upon, which is then extinguished. Hippocrates wrote about two things relating to sustenance: “sleep and fasting make the body thin and cold, and they remove the moisture it contains; but lengthier periods of these will heat up, melt, and wear out the flesh.”

Some physicians have written that when a person works hard during the day, his body and blood deteriorate, his essential humors decrease, and he loses much of his pneumata. Therefore, the Creator causes a deep sleep to fall upon him [see Genesis 2:21], through which he is strengthened, his moisture ripens, his natural heat increases, his flesh is fattened, and the worries that trouble his thoughts and heart are eased. This also helps the digestion of foods and provides support for weakened limbs. Nevertheless, one must avoid lengthy sleep and take active measures to refrain from this practice, as it cools and weakens one’s natural heat, produces evil moistures, and causes vapors to rise up to the head. It is especially harmful to portly individuals, as it causes the retention in the body of those superfluities that should be expelled.

Regarding sleep, some maintain that its purpose is to cause vapors to rise from the food to the brain, and as one is cold they harden there and become heavy, and then upon their descent they close the pathways of the senses until the heat dissipates them, at which point the person wakes up. Sleep is very beneficial for one whose predominant humor is black bile, for it promotes heat and moisture, which is the opposite of his nature.

One should avoid sleeping in the daytime, apart from someone who is accustomed to doing so a little, or if one did not sleep at night, or if he senses that his limbs are weak, or that his innards are enfeebled. In such circumstances, one may sleep during the day on a chair, while keeping his head upright. The reason why one should not sleep by day, especially in the summer, is because the heat of the season draws out one’s natural bodily heat while sleep is gathering it inward, and thus two opposite movements are occurring simultaneously. Daytime sleep can lead to many illnesses, such as the descent of phlegm from the brain to the chest, and it also distorts one’s countenance, causes a heaviness in the spleen, and damages the sinews.

At night, one should not sleep with an uncovered head, and should not go to sleep immediately after eating, but should wait two or three hours, by which time he will have properly digested his food. It is also harmful to sleep in the daytime when one has fasted and his belly is empty of food. One should not turn over a lot in bed, as this upsets the digestion, and one should not sleep on his neck, because this can damage the spine and the kidneys, since the waste product will descend to the spine and block one’s breathing.

At the beginning of the night, one should sleep on the left side, so that part of the liver can get closer to and rest on the stomach, and the food will be warmed by its heat. At the end of the night he should turn to his right side, in order that the food substance should easily descend to the liver while the waste product will go to the intestines. One should not sleep on his face, so that the superfluities and the humors do not descend to the eyes. One who has a feeble belly should sleep on his belly, in order to heat it and thereby aid the digestion.

Some claim that the proper amount of sleep depends on the digestion of food, and the sign for this is one’s urine—when it is clear as water, this is an indication that the food has not yet been processed, and therefore he should sleep more. When the urine is yellow, this is a sign that the food has been processed, at which point he should wake from his sleep and arise from his bed. And some contend that for one who is naturally warm, like a young man, six hours is sufficient sleep, whereas one who is cold by nature, such as an old man, should get eight hours. The sign that a person has slept enough is lightness of the body, especially the brain, and that the food has descended from the stomach to the intestines, and he has the urge to attend to his natural functions and to urinate, as well as a settled mind and heart from all his agitation. In light of all the above, one can fully understand what our master wrote in this section with regard to sleep.