590 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 



while another who has no fixed time for retiring does not feel the need 

 of it at any special hour. Some, again, can not sleep in daylight, 

 some heartily enjoy long morning naps, while others work by night 

 and sleep through broad daylight. The best proof that a special time 

 for sleep is merely a habit is that we can readily change the hours. 

 If I wish I can pass tins night without sleep, and likewise, if such be 

 my desire, I can go to sleep as soon as this lecture is finished. An 

 experiment may oblige us to pass one or even two nights without 

 sleep, and without affecting the accuracy of our observations. A 

 physician is awakened for an urgent operation; he performs it without 

 a mistake. Who has not in mind the story of entombed miners look- 

 ing for deliverance many days without ceasing? Not only can we 

 sleep or stay awake at will, but we can regulate the duration of our 

 slumber. Most persons sleep eight hours daily, but we can sleep longer 

 if we make it a habit, and much less if we need to do so. Napoleon 

 slept only three or four hours a day. What better example of habit is 

 there than that of some children who can go to sleep only after the 

 mother's good-night kiss, and that of persons who can sleep only in a 

 certain position ? 



But our sleep is not always due to habit. The sight of others asleep 

 near by, a monotonous or even a varied noise, have put us to sleep, 

 without our thinking of it. So some persons fall asleep at their 

 studies, at lectures, and even at the opera. The process of digestion 

 and fatigue certainly favor sleepiness, but its true cause is indifference. 

 To yawn, to doze, are signs of weariness. "They doze," says Bergson, 

 "in the exact proportion to their lack of interest in their surround- 

 ings." 



However, one must not believe that sleep is either merely a habit or 

 an act of indifference alone for it is equivalent to a necessity, and we 

 could not depri"s*e ourselves of sleep without serious consequences. 

 An animal deprived of sleep dies after a few days, much sooner than if 

 it were deprived of food. We generally sleep longer and much 

 sounder after prolonged wakefulness, when our attention has been 

 most sustained. Sleep refreshes, it revives. If it is an instinct, a 

 habit, it is an excellent one, and we would not know how to do 

 without it. 



You know the signs that are forerunners of sleep. The first is a 

 sensation of pricking or tingling of the eyes, winch is explained to 

 little children as the coming of the sandman. Then we begin to 

 gape, the head grows heavy, the lirnbs weary, the eyes close, atten- 

 tion vanishes, the head bends, and we doze. For a time we still are 

 conscious of the tilings about us, but presently the sense of smell and 

 of touch are gone, the hearing weakens. With these last sensations 

 are often mingled shapeless dreams, with little intensity, incoherent, 

 then all ceases, our consciousness is gone, we sleep. Sleep varies in 



