Page:Popular Science Monthly Volume 63.djvu/436

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T is not easy to define the condition of sleep in terms that will not admit of many exceptions. We readily recognize the states of rest and activity, but where the element of consciousness must be considered we are at once upon uncertain ground. If we think of sleep as an unconscious state, sharply contrasted with waking, we do well to limit our use of the word to the case of man and the most intelligent animals. Sleep in this sense is only to be associated with highly developed nervous systems and its final explanation is to be sought in events taking place in the brain.

Various writers upon the subject of sleep have turned their attention to quite different aspects of the matter. Some have undertaken to show why there is the need of sleep and why the tendency to sleep comes on at the close of each day. These writers have dealt with general or systemic causes. Others have concerned themselves with the cause of the unconscious—or dreaming—state in which the sleeper lies. They have endeavored to suggest intimate and local causes. Since the several theories are thus distinct in their application they are not necessarily mutually exclusive.

Broadly speaking we feel sure that the need of sleep follows from general or local fatigue. During waking hours the decomposition processes of the body doubtless rise above the life-long mean, and sooner or later there must be a compensatory fall below the average. The adaptation of the race to alternating light and darkness has made this rhythmic rise and fall to coincide with day and night—though less rigidly under the artificial conditions of civilized life than in more primitive times.

Fatigue at bottom is a chemical phenomenon, and so the theories of the first class are chemical. When a muscle has been stimulated until it exhibits the well-known signs of fatigue, there are two possible inferences—either this means an exhaustion of fuel substances or an accumulation of poisonous waste. Analogous views have been supported in regard to the chemical changes that lead to sleep. We have had an exhaustion theory advanced by Pflüger and an accumulation theory offered by Preyer.