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!” In this tendency to nightly inaction man shares with all other living creatures. His body thus testifies to the intimacy of its relations with all portions of the solar system. Originated in the tropical regions of the earth, where day and night are nearly equal, we find in all parts of the world the same hereditary need of a period of rest, nearly coincident with the duration of the shorter nights of the tropical year. Had the birth-place of primeval man been situated within the Arctic circle, it is probable that his hours of sleep might have differed considerably from the number now needed by the average individual. So powerful are the necessities thus dependent upon the harmony between our organization and the movements of the earth, that if the habit be formed of sleeping at other hours than those which are usually devoted to that purpose, the full complement of sleep is still needful to satisfy the demand for rest.