All that the writer aims to do is to set forth the best theories of how to insure sound sleep and good health, and to leave it to the individual reader to try whichever he thinks fits. It is what he will do, anyway, if he is a wise man; for only by following the course he most desires can he learn whether these desires are to be trusted as guides to happiness and well-being. But—most persons eat too much or too often or too fast. Maybe you do, too. CHAPTER XXVI SOME MODERN THEORIES OF SLEEP I have an exposition of sleep come upon me. SHAKESPEARE. There have been almost as many theories of sleep and its causes as there have been investigators, but these theories may be grouped under a few main heads: Physiological, or that which has to do with some bodily conditions only, and which made men think that sleep was dependent upon the circulation of the blood or upon decreased consumption of oxygen, was an early one of these theories. It has had many advocates and has led to many interesting experiments that have increased the sum of general knowledge, although they have not explained sleep. Delicate instruments, with formidable names, have been invented and successfully used to measure the intensity of sleep and to note its phenomena. Two of the experimenters—C. E. Brush, Jr., and R. Fayerweather of the Physiological Laboratory of Johns Hopkins University—through long, intricate and exhaustive experiments, have found that sleep is most intense and the pressure of blood in the arteries lowest during the first half of the sleeping period. After we have completed the first half of our sleep, the intensity or soundness of sleep becomes less, and the pressure of blood in the arteries continues to increase up to the moment of awaking. It is interesting to learn that the moment when we are most soundly sleeping is at the end of the first hour of sleep, and that the blood-pressure has at that time reached its very lowest point. Messrs. Brush and Fayerweather report that, during the first few hours of sleep, the blood-pressure continues to fall and then begins a gradual rise. The tendency is to more and more rapid flow of the blood, but this rise is not steady or regular, because it is broken by long waves when the force of the circulation falls and the pulse is weaker than it was a moment or two before. The rapidity of the blood-flow is greater on the moment of awakening than just before dropping to sleep. This increase is not sudden, but is the culmination of the rise that begins a few hours after we fall asleep. (See Appendix B.)