That the depth of sleep is exceedingly variable is evident in the experience of every one. A German physiologist[17] has made a rough estimate of the soundness of sleep by comparing the loudness of the noises necessary to wake the subject of experiment at regular intervals during the course of the night. He arranged a gong with a pendulum attachment, and noted the length of the stroke which produced a sound sufficiently loud to awaken the patient. In this way the different degrees of intensity of the awakening noise could be calculated, and the corresponding depth of sleep could be estimated. It was thus concluded that the depth of sleep increases rapidly during the first hour, at the end of which time it has reached its maximum. During the next half hour it diminishes as rapidly as it had increased in the first half hour. During the next hour it still further diminishes, almost as much as it increased during the second half hour. The remaining ten half hours of the experiment were occupied by a comparatively light and gradually diminishing slumber, until the vanishing point of sleep was reached at the expiration of eight hours from its commencement. This observation corresponds with the general opinion that sleep is deepest in the early part of the night. For the same reason dreams and wakefulness are most frequent during the early watches of the morning.
When considering the causes of sleep it is needful to exclude from view those artificial varieties of sleep that are produced by the various narcotic drugs, as well as the counterfeits of sleep which result from diseased conditions of the body. It is comparatively easy to frame hypotheses in explanation of such interruptions of our conscious life; but, when we attempt to formulate a theory which shall satisfactorily account for the occurrence of natural sleep in healthy animals, the task becomes exceedingly difficult.
First among the causes of sleep may be reckoned the alternation of day and night. With the disappearance of sunlight all nature sinks into a condition of repose.
“The night brings sleep To the greenwoods deep, To the bird of the woods its nest; To care soft hours, To life new powers, To the sick and the weary--rest!”