Why Do We Go to Sleep?
First, of course, we sleep to rest our body and brain. During our waking hours many, if not all, parts of our bodies are active all the time, and with every movement we exhaust or spend some of our strength. Take the case of your arm, for instance. You may be able to move it up and down fifty or a hundred or more times without getting tired, according to how strong you are, but sooner or later you will not be able to move it any more—it is tired—the life has all gone out of it and it needs rest, in order that it may become strong again. Every time you move your arm you destroy certain parts of its tissues, which can only be replaced during rest. Every activity of your body has the same experience, and the constant work of the brain in directing the various movements and activities of the body, tires it out too. As soon as this condition occurs, the brain tells the other parts of the body that it is time to rest, and even if we try to keep awake and go on with our work or play, or whatever it is we are doing, we find sooner or later that it is impossible. If we persist we fall asleep wherever we happen to be. It is not necessary for all parts of the body to be tired before we sleep. One part alone may be so affected by what it has been doing that it alone causes us to fall asleep. Sometimes the eyes become so tired, while we are looking at the pictures in a book or reading, for instance, that we fall off to sleep quickly. It is perhaps easier to bring on sleep by making the eyes tired than in any other way. That is why so many people read themselves to sleep. It is such a gradual passing into unconsciousness that you can hardly ever tell where you left off reading. It is said that when we are awake our bodies are continually planning for the time when we shall need sleep and are continually making some little germ which is carried to the brain as soon as made, and when there are a sufficient number of these little germs piled up in the brain, we go to sleep. The process of sleeping then destroys these germs, and when they are destroyed we again wake up.