592 ANNUAL. KEPORT 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: "Sleep is a function of the vital 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 Manaceine, that it is the period of repose 

 of our consciousness; with Bergson, a reaction from indifference; 

 with Claparede, 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. All 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 anaemia 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 be a partial cause of perspira- 



