592 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
This observation that anyone can make leads us now to examine 
some of the definitions of normal sleep. 
We may say with Barthez: peice: is a function of the eu) prin- 
ciple alternating with being awake,” or, better, a vital function alter- 
nating with being awake. With Sergueyeff, we recall that it is nec- 
essary and periodic; with Manacéine, that it isthe period of repose 
of our consciousness; with Bergson, a reaction from indifference; 
with Claparéde, that it is an instinct or a habit. All these definitions 
are exact and their resemblance characterizes the phenomena clearly 
enough. But if we would go further and know why we go to sleep, 
we must not only observe a man asleep, but more than that, we must 
analyze the action of various bodily functions during sleep. 
Making experimental observations of sleep is peculiarly difficult. 
There is great risk of disturbance merely through one’s presence, by 
noise, or on account of apprehension or irritation caused in the sleeper 
by the pressure of the instruments resting on him. Although there 
are difficulties almost impossible to overcome, yet physiologists have 
succeeded on several occasions in making important experiments upon 
sleeping men. 
First of all, certain observations require no instruments, such as 
measuring the number of respirations or the beating of the heart. 
Then one can become accustomed to going to sleep while holding an 
instrument which will register various movements produced during 
sleep. Sometimes you come across cases where, as the result of a 
fall or from a surgical operation, there is an opening in the skull 
exposing the brain. By observing such cases we can actually see 
what happens to the hidden organs during sleep. AJ these methods 
of observing have resulted in establishing certain data that I will 
briefly review. 
Digestion goes on effectively during sleep. The evening meal is 
digested during the night. The midday meal, especially when it is 
heavy, causes drowsiness in some persons. The proof that digestion 
is active in the night is the fact that waste is generally accumulated 
in the morning. Furthermore, in the case of persons who have died 
during the night the autopsy shows that digestion is further advanced 
the longer the period intervening between the last meal and death; - 
from this fact the probable time of death has been indicated at 
coroner’s inquests. The activity of the stomach indicates the cause of 
the anemia of the brain which induces sleep. To the hygienist, 
digestion during sleep solves the problem of determining whether it is 
better to go to bed immediately after a repast or to wait several 
hours until digestion has commenced. 
Excretory functions continue during sleep. It is a well-known 
fact that perspiration is then active and that the bladder is full in 
the morning. The heat of the bed may bea partial cause of perspira- 
