Studying sleep, we note that the breathing is much deeper, and more forcible and rhythmic than in the waking state. Much more air is inspired when asleep than when awake, and this tells us that the Principle of Health requires large quantities of some element in the atmosphere for the process of renewal. If you would surround sleep with natural conditions, then, the first step is to see that you have an unlimited supply of fresh and pure air to breathe. Physicians have found that sleeping in the pure air of out-of-doors is very efficacious in the treatment of pulmonary troubles; and, taken in connection with the Way of Living and Thinking prescribed in this book, you will find that it is just as efficacious in curing every other sort of trouble. Do not take any half-way measures in this matter of securing pure air while you sleep. Ventilate your bedroom thoroughly; so thoroughly that it will be practically the same as sleeping out of doors. Have a door or window open wide; have one open on each side of the room, if possible. If you cannot have a good draught of air across the room, pull the head of your bed close to the open window, so that the air from without may come fully into your face. No matter how cold or unpleasant the weather, have a window open, and open wide; and try to get a circulation of pure air through the room. Pile on the bedclothes, if necessary, to keep you warm; but have an unlimited supply of fresh air from out of doors. This is the first great requisite for healthy sleep. The brain and nerve centers cannot be thoroughly vitalized if you sleep in "dead" or stagnant air; you must have the living atmosphere, vital with nature's Principle of Life. I repeat, do not make any compromise in this matter; ventilate your sleeping room completely, and see that there is a circulation of outdoor air through it while you sleep. You are not sleeping in a perfectly healthy way if you shut the doors and windows of your sleeping room, whether in winter or summer. Have fresh air. If you are where there is no fresh air, move. If your bedroom cannot be ventilated, get into another house.