In spite of the fatigue often experienced before we retire, we awake again on the morrow with renewed strength and power. From these and other reasons we are led to believe that during the hours of activity the body is constantly expending vital energy in both internal and external work, and that during the hours of sleep, through some unknown process, the body is charged with vital energy which is stored up and used gradually for carrying on the various functions and activities of the system. [Sidenote: The mystery of energy] Just what this energy is, just where it comes from, just how it is stored, just the manner in which it is delegated to the body, we cannot say. We can only observe its workings, or effects, and formulate therefrom a theory. We are led to believe, however, that this energy is stored in the nervous organism, perhaps most largely in the brain, as brain tissue is the last to break down or waste away in sickness, ill health, dis-ease, or starvation, often maintaining its full weight up to the point of death. [Sidenote: Vital processes expend energy during sleep] Even in sleep the expenditure of energy in the vital processes continues vigorously, depending upon conditions immediately preceding sleep, but usually in a much more passive degree than in the waking hours. These activities, however, are no more pronounced in their constructive action or repair, than in ordinary periods of rest during the waking hours. SOME REASONS [Sidenote: Food furnishes but a fraction of the total body-energy] The processes of nutrition, alone, demand the expenditure of much energy, and the degree of energy available from foods, even by perfect combustion, would yield but a fraction of the energy expended by the body. [Sidenote: Energy required for work in excess of energy obtained from food] The average laborer in shoveling coal, swinging an axe or a pick, expends energy far in excess of the amount that could possibly be obtained from his food. A day laborer may eat a piece of beefsteak, two or three potatoes, and a few slices of bread, and will shovel twenty tons of earth to a height of five feet; a Japanese soldier will carry a heavy load and walk all day, subsisting only on a handful of rice, and besides this, will do some thinking, which consumes energy. [Sidenote: Evidence gained from "fasts" and "no breakfast" plan] We also have on record fasts, of from thirty to forty days, which, in some cases, show a slight gain in strength. There are also hundreds of students of natural living who adopted the "no breakfast plan" and again many, only one meal a day, limiting their consumption of food to comparatively small quantities of nuts, fruits, and vegetables, who have found thereby a remarkable increase in vitality, strength, and general physical and mental power. [Sidenote: Relation of sleep to expenditure of energy]