Prominent among the causes which predispose to sleep at night is the cessation of a majority of the sensations that are continually pouring in upon the brain during the period of daylight. Hence the necessity for seclusion in darkened rooms, from which the noises of the daytime are shut out, if one would sleep during the long days of the arctic summer, or if one would enjoy a midday nap at any season of the year. The close dependence of wakefulness upon the constant activity of the organs of sensation, is well illustrated by a case related in Hermann’s _Handbuch der Physiologie_, Vol. II, Part 2, p. 295. A young man had been reduced by disease to such a condition of general anæsthesia that the right eye and the left ear were the only remaining paths of sensation between his brain and the external world. Whenever the sound eye and ear were bandaged so as to cut off all communication with the brain, the patient invariably fell asleep in the course of two or three minutes after the interruption of sensation. In like manner, some people, even in perfect health, are able to sleep at any time by simply lying down and closing the eyes. Such persons, however, are not often very highly gifted in the intellectual sphere. They generally belong to a class of men whose lives are laborious and liable to great irregularity and fatigue. Such people labor in the open air, where every organ of sense is in a state of continual excitement. As soon, therefore, as they can find a quiet corner from which the commotion of the elements is excluded, it is only necessary to close the eyes--the principal avenue of communication with the outside world--and sleep begins at once. This is especially true if severe bodily exertion has preceded the opportunity for repose.