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.
OXIDATION AND AIR
Relative importance of air, food and water
One of the most important of the vital functions is breathing. Physiologists, teachers, and lecturers continually remind us of the comparative time we could live without food or water, and the remarkably short time we could live if entirely deprived of air.
Oxygen not the only required element in breathing
Oxygen is vitally necessary for the purpose of purifying the blood and supplying the various tissues and fluids in the body, of which oxygen forms an important constituent. However, oxygen is not the only necessary element which is utilized by the system in the process of breathing, as human beings die immediately upon being placed in a receptacle of undiluted oxygen. Just what this other factor is, science has not clearly defined, but that it is concerned with rest and sleep we have at least unconsciously recognized, as shown by our often referring to periods of rest as "breathing spells"; from the fact that we have found it of great importance to keep the air we are breathing moving constantly about us, especially while asleep. From all these facts we are forced to believe that sleep plays an important part in producing and maintaining body-energy, besides constantly recharging the system with oxygen.