The intensity or depth of sleep is shown by a curve that looks like a pile of sand with the top scooped off. It increases rather slowly, in most cases, for the first quarter-hour: then quickly, so that half an hour later the person is most “sound asleep.” He stays so, on the level top, for about half an hour. That is the time that wise burglars and late husbands choose to steal into the house, about an hour after everyone is asleep. After that time the sleeper reverses the process of falling into deep sleep by getting nearer to waking for half an hour and then getting, at first rapidly, nearer to waking for two or three hours. In the last three or four hours healthy and normal persons reach about the same proportions of time and intensity of sleep, so that the Indian-bow-shaped curve fairly represents how long it takes everybody to deepen his sleep. Kohlschütter found how great an intensity of sound was needed to awaken a sleeper at different periods throughout the night. His curve thus made tallies very exactly with that of Brush and Fayerweather, obtained in quite a different way. Some other investigators have pointed out that, interesting as this theory is, it proves one thing about as completely as it does the other. For, while it is plain that sleep and the great fall in blood-pressure exist at the same moment, it is not conclusively shown which is cause and which is effect. Does sleep cause the fall in blood-pressure, or does the fall in blood-pressure cause sleep? The two are coexistent, but who can say which begins first? It looks as if sleep might be more justly considered the cause, if one takes the sleeping-position, and maintains the attitude of mind suitable to induce sleep, blood-pressure grows less, even though the patient does not actually fall asleep. Under this physiological view must come also the chemical theory based on the fact that we consume more oxygen during the day, thus forming carbon dioxide and other poisons which cause sleepiness. During the night we absorb oxygen, building up the tissues, and eliminating the poisons of the waking hours. The poisons which are the result of the consumption of oxygen cause fatigue, and according to Preyer, a European authority, “sleep is the direct consequence of fatigue, or rather of the fatigue products in the blood.” His contention is that, if lactic acid and other chemical products of the consumption of oxygen in the body were injected artificially, sleep would follow. Experiments in this direction made by Preyer, Fisher, and L. Meyer have yielded such contradictory results that the theory is not proved thereby.