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 this 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 mterest in their surround- 
ings.’ ; 
However, one must not believe that sleep is either merely a habit or 
an act of indifference alone for 1t is equivalent to a necessity, and we 
could not deprive 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, which is explained to 
little children as the coming of the sandman. Then we begin to 
gape, the head grows heavy, the limbs weary, the eyes close, atten- 
tion vanishes, the head bends, and we doze. For a time we still are 
conscious of the things 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 
