Page:First course in biology (IA firstcourseinbio00bailrich).pdf/599

 even to awaken has great presumption. If one does not rise promptly upon waking naturally, the instinct to awake when enough sleep has been taken will be lost, and the habit of sleeping too much will be formed, and the brain, like the muscles, will become weak from inactivity. Infants sleep most of the time, and it is injurious to them to be waked. Adults usually require about eight hours of sleep. There is a risk in going to sleep in a warm room, for the bed covering which is comfortable then may not be enough to prevent taking cold when the fire goes out. Sleep usually comes more promptly to one who goes to bed at the same hour each night. The muscles are relaxed in sleep, and relaxing them perfectly upon lying down and breathing slowly, tends to bring sleep. One who is sleepless usually finds that he is breathing fast and is holding the head stiff on the shoulders, the teeth clenched, and the muscles contracted, even though he is lying down. Excitement and worry during the day, but especially just before retiring, tend to produce sleeplessness. One who overworks his mind by too great attention to business is inviting ruin. A student who loses sleep while preparing for an examination will probably fail. Rested brain cells and pure blood are needed for good work.

Rules for Preventing Sleepiness.—(1) Do not sit close to stove or especially a fireplace or in very warm room, and do not wear very warm clothing in the house. (2) Let in fresh air freely. (3) Do not sit in rocking chair nor with chest flattened. (4) Make the last meal a very light one.

Habits.—Our habits of doing and thinking and feeling really constitute our characters. This shows the importance of right habits. By gradually changing our habits we can strengthen our characters and form them somewhat as we wish. When a muscle contracts in a certain way, this act makes it easier for the muscle to contract in that way the next time; thus great muscular strength may be developed. When a nerve cell acts, the circulation around the cell is increased, the fibers develop by use, and the act is easier the next time. We cannot entirely get rid of our habits, because we cannot get rid of our brains.

Healthy fatigue is caused by the accumulation of waste products resulting from the oxidation of substances in nerve, muscle, and gland cells. The presence of waste in