There are two kinds of sleep: (1) beneficial sleep; and (2) harmful sleep. Beneficial sleep restores one’s adaptive capacity, while exertion and lack of sleep causes chemical changes in the body and may effect lethargy and hallucination. Sleeping is necessary for the body. During sleep, the basal metabolic rate and body temperature drop and allow the body to rebuild its needed chemical balance, restore any chemical deficit, and to regain its control upon waking up.
Hence the body needs rest, and there are two kinds of it: (1) natural rest; and (2) unnatural rest. As for the natural rest, it involves the deactivation of the natural human forces, including the senses, and control of body movements. During wakefulness, body moisture and vapors dissolve and burn up due to physical movement and exercise. Once such forces cease their activities by relaxing, body moisture and vapor rise to the brain, which is the source and motor behind every movement. Consequently, one will fall asleep. As for the unnatural rest, it is usually due to an illness or a weakness. In this case, body moisture will increase, obstruct normal brain activities, and cause dolefulness, loss of vitality, and excess sleeping. Hence, even mere wakefulness cannot burn up the excess moisture and vapors in the brain. A similar condition of fatigue occurs upon overconsumption of food, whereby heat rises in the stomach during coction, and when the digestive enzymes cannot break down the polysaccharides into their constituent monosaccharides, and when they enter the blood and are transported to the brain, such excess sucrose can cause disturbance in brain function, obesity, depression, even to the point of coma. Obesity also can cause elevation of blood fat, and can increase cardiovascular risks. Therefore, excess sleep is not natural.
However, sleep is needed to provide two essential benefits for the body: (1) by relaxing, the body regains its strength, energy, and the natural physical range of normal functions that will otherwise be difficult to control; and (2) sleeping helps every organ to dispose of its waste products. In fact, during one’s sleep, enzymes, intestinal juices, gastric juices, bile salts, and pancreatic juices flow more easily, and particularly when the brain ceases to conduct the normal functions of wakefulness. The concentration of such natural process in the stomach is noticed by the dropping of body temperature, and the lowering of the basal metabolic rate. This is why one feels the need to be covered when resting.
Moreover, the stomach tilts slightly downward towards the small intestines, and to ease the process of digestion, it is healthier to sleep on the right side. On the other hand, sleeping regularly on the left side can disadvantage the heart, because it will throw pressure from the other organs upon it, while it is unhealthy to regularly sleep on the back. However, it will cause no harm to rest on it without sleeping. Finally, sleeping on one’s stomach is most damaging to one’s health. On this subject, it was narrated by Abi Umama that God’s Messenger صلى الله عليه و سلم saw a man sleeping on his stomach in the Mosque. He woke him up and reprimanded him by saying: “This is the lying down posture of the dwellers of hell-fire!” (Reported in Sunan Ibn Mãja). Sleeping on the stomach without a good reason does not look natural, and it also could represent mental confusion. However, if one does sleep on his stomach because of stomach pain, this may not be as damaging as long as it does not become a habit.
A balanced rest helps the body regain its strength, refreshes one’s spirit, and brings out one’s adaptive capacity to its fullness, and by compensating for one’s energies, sleep will achieve coherence and prevent slackness of one’s spirit. Sleeping during the daytime can breed rheumatic diseases, cause deficiency in the color of one’s skin, cause inflammation of the spleen (splenitis), weaken the nervous system (neurasthenia), and weaken sexual desire except for sleeping in summertime during midday heat. The worst of daytime sleeping is to sleep during the early hours of the day, and even worse than that is sleeping in the mid-afternoon during the latter hours of the day and before sunset. ‘Abdullãh bin ‘Abbãss once found one of his sons sleeping late in the morning, and he woke him up saying: “Wake up! Why would you sleep during the hour when people’s earnings are being apportioned?” Some of our predecessors also said that daytime sleeping is of three kinds: (1) natural; (2) agonizing; and (3) idiotic. As for the natural, it represent God’s Messenger’s صلى الله عليه و سلم occasional short nap in the early morning. The second kind is to sleep late in the morning, hence depriving one from the benefits of this world and the next. Sleeping late in the morning also may cause one to inherit general body weakness and malaise. Finally, the third kind is to sleep in the mid-afternoon and before sunset, and that may cause dullwittedness.
There is another disadvantage and health hazard associated with late morning sleep, and except for being bedridden, sleeping in the mid-morning can cause the putrefaction and foulness of body wastes which are otherwise discarded due to body movement and exercise in the morning. Such putrefaction can cause damage to tissues and the breakup of some internal vessels. Otherwise, and without eliminating waste matters from the bowels and engaging the stomach with a light breakfast, and should it develop into habit, this could become the cause of various incurable diseases.
On the other hand, sleeping regularly under the sun can cause cancerous growths and affect the epithelial cells besides causing other hidden diseases. Sleeping in partial sun and partial shade is also unhealthy. Even merely sitting partly in the sunlight and partly in the shade is bad for one’s health. This is noted in God’s Messenger’s صلى الله عليه و سلم saying: “If you are sitting in the shade and the shadow recedes partly by the sunlight, then move to another place” (Reported in Sunan Ibn Mãja). This is another indication that one should not sleep in the sun, or even partly in the sun.