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.)

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.

The idea that sleep is the result of poisons in the system takes us into the pathological theory of sleep, which regards it as a sort of disease like epilepsy or auto-intoxication. We produce by our own activities the poisons which cause insensibility until the system cleanses itself. Professor Leo Errera of Brussels says that “work in the organism is closely bound up with a chemical breaking down.” Among the products of this breakdown are “leucomaines,” the scientific name for poisons formed in living tissue, and just the opposite to “ptomaines,” which, however, are also virulent poisons.

Professor Errera tells us that, during our waking hours, we produce more leucomaines than the oxygen we absorb can destroy. This excess is carried along by the blood and held by the brain centers, and in time produces sleep, just as any poisonous anÆsthetic, such as morphine, would produce sleep.

While we are sleeping we absorb much oxygen and we recover from the effects of our self-intoxication. Errera maintains that work, fatigue, sleep, and repair are not merely successive events, but phenomena chained together in a regular and necessary cycle. He explains sleeplessness due to overfatigue on the theory that small doses of poisons induce sleep and large doses induce excitement and even convulsions.

ManacÉÏne points out that this theory is good from a purely physical standpoint, but does not explain our power to postpone sleep or the faculty of waking at a fixed hour. We can do both, and any adequate theory of sleep must explain why we can control the tendency to sleep, but cannot control the symptoms of ordinary poisoning.