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 pure, nutritious blood. Nearly one fifth of the blood goes to the brain. It is clear that the brain cannot give out energy until it has first received it; the blood supplies energy to the brain. The blood in turn receives the nourishment from food and pure air. A rested cell is full of nourishment; a tired cell is shriveled (see Fig. 117).

A, resting cell, B, fatigued cell, with its body and nucleus shrunken.

Sleep.—During waking hours energy is used up faster than it is stored in the cells, and protoplasm is oxidized faster than the cells can replace it. During sleep the opposite is true; repair is more rapid than waste. During sleep the muscles are strengthened, the breathing is less, the heart beats more slowly, less heat is produced, digestion is slower, less blood goes to the brain. Why is it necessary to be more warmly protected by clothing or bed covering when asleep than when awake? Above all, the nervous system has an opportunity to recuperate from the constant activity of waking hours. The eye and the ear are rested by darkness and silence. Sleep caused by morphine or other drug is not normal sleep and brings little refreshment.

Practical Suggestions.—Sleep is deepest during the second hour after going to sleep, and a greater shock is given to the nervous system by waking a sleeper during that hour than at another time. An alarm clock is a very unhealthful device. One who cannot trust to nature