SOME REASONS
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.
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.
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.
Relation of sleep to expenditure of energy
Since the processes of nutrition, including digestion, circulation, assimilation and excretion consume energy, and notwithstanding this we are able to perform hundreds of foot-tons of labor a day besides; since we have found it possible to continue to live, and in some cases to even increase the amount of strength and work-power on a very limited diet; since it is a mathematical impossibility to produce as much energy from the food consumed as the body expends, we are forced to the conclusion that we do not obtain all our energy from food. Therefore, from a careful analysis of the phenomenon of sleep, we conclude that it is very closely connected with this mystery.