This section is from the book "Health Without Medicine. A Treatise On The Laws Of The Human System", by Larkin B. Coles. Also available from Amazon: Philosophy Of Health.
Time for labor, taken in its relation to the time of taking food, makes an important item in the scale of means for preserving health. This matter has been considerably anticipated and superseded while considering the subject of exercise; yet something more may be said, and the substance of previous remarks reiterated, so as to leave no chance for misapprehension or forgetfulness on this subject.
Labor is intended to mean close and intent application to business, whether of a bodily or mental character. No labor should be attempted while the nervous system is intensely engaged in the process of digestion. And as the time of this intensity is during the first hour after a meal is finished, no labor should be performed during that hour, If a laborer commence hard work immediately after eating, the action of his nervous energies is distracted; partly drawn toward the stomach, and partly forced in the direction of the muscular system. By this unnatural, forced action of the nerves, the digestive process is impaired; the food is not thoroughly broken up by, and mixed with, the gastric juice. By this unnatural operation, the food is comparatively unprepared for all the rest of the process. The chyme and chyle must be imperfectly formed, and the system, so far as each such meal is concerned, imperfectly nourished. Besides this, the forcing of the muscles to exertion against the natural inclination of the nerves to supply the necessary power, gradually impairs the power and activity of the muscular system.
The man who disregards this law will grow old faster -- other things being equal -- than the man who allows time for the thorough digestion of his food. It is his food which sustains him in labor; therefore, he is in duty bound to give that food the best possible opportunity to give him support. The same law prevails in dumb animals as in man. Whoever works his oxen or drives his horses immediately after their eating, will find, in the course of an experience sufficient to test the point, that his beasts, under such a management, will soon wear out; while his neighbor's beast, under a management which accords with nature's law, will be robust and endure. It is economy, then, as well as health, to yield obedience to this natural law.
Mental labor should never be attempted within one hour after a meal is finished. If a close mental application be made immediately after eating, whether at be a merchant casting accounts, or a student getting his lesson, the digestive process is impaired; the nervous energies are drawn, in a measure, away from the direction of the stomach to the brain. This unnatural action frequently causes an increased quantity of blood to be lodged on that organ, occasioning a dull, heavy headache. Sometimes it will bring on a nervous headache. The influence of this course is also very injurious to the stomach. Hundreds and thousands of students have in this way brought upon themselves dyspepsia, with its long train of untold symptoms and sufferings. Many a one has in this way broken irremediably his constitution. With too little physical exercise at the right time, and with mental labor at the wrong time, he has ruined himself for life, or brought himself to a premature grave. Many a one has gone through a regular course of education -- prepared his mind for usefulness -- but by having neglected the laws of his body -- neglected to keep up a proper balance of action between his physical and intellectual powers- -- he has rendered himself disqualified for much execution in the callings of life. His mind, though well disciplined, cannot act in this life without a body; the bodily energies are so deranged and weakened, as to hold the intellectual faculties in a state of comparative imbecility.
Students should accustom themselves to considerable daily exercise of body, in order to preserve a balance of physical and mental energy. This should be done for the sake of aiding them in making intellectual proficiency, and of preserving a good constitution for future usefulness. Their principal physical exercise should be taken on an empty stomach, i.e., just preceding a meal. Just after a meal, they should be at leisure, or amusement which requires no mental or physical exertion, for at least one hour. Then they are prepared for close study until near the time of the next meal; leaving a little space for relaxation: as also when bodily exercise precedes a meal, a few minutes' relaxation before eating should be had, that the nerves may regain their equilibrium. But when exercise is spoken of in relation to students, that which would agitate or exhaust the body is not meant. Such exercise would be decidedly detrimental. If students would give time for eating and for digesting, they could perform a large amount of mental labor with far less time devoted to mere exercise, and that exercise of a milder character, than would otherwise be required. But every student should accustom himself to a brisk, lively, cheerful daily exercise, if he values his health. The same rule applies with equal force to every one, whatever may be his calling, whose labors are of a mental character. Under these rules, three hours of close study would be worth more than six in the ordinary way.