Shakespeare writes of "Sleep that knits up the ravelled sleeve of care." The metaphor is striking, but not accurate. To knit up that which is ravelled implies using the old material in repairing the damage, but that is not the way in which the body is rebuilt. The old material is thrown out and new material put in its place, and that largely takes place during sleep. We have read of brownies who came at night and swept and churned and baked while the housewife slept. So, in our bodily dwelling, the vital forces are our brownies, and they can work more uninterruptedly while we are asleep than when we are calling on them to move us from place to place, or to aid us in various activities. Much of life's processes must remain a mystery to us, but certain things we have learned, and one is that perfect health cannot be maintained, strong nerves cannot be constructed, nor a clear brain be built without plenty of sleep. The baby sleeps almost continually because he is building so much new structure. The growing child needs more sleep than the adult; but even after reaching maturity sleep cannot be materially lessened without injury to the whole organization. We appreciate the need of food. We are often very needlessly alarmed for fear that we shall starve from one meal to the next, but few of us realize that food cannot be assimilated, built into tissue, without some hours in which the vital forces can devote themselves wholly to the work of assimilation. During the working hours of the day we are expending force. The brain is using it in thought, the muscles are calling for force in various activities, the emotions are expending energy, and each of these activities is creating changes in the cells of the body. We know that life in the body is only possible through constant death of the atoms of which it is composed. We can only live because we are constantly dying. Huxley says, "For every vital act, life is used up. All work implies waste, and the work of life results directly or indirectly in the waste of protoplasm (which is the cell substance). Every word uttered by a speaker costs him some physical loss, and in the strictest sense he burns that others may have light."