CHAPTER XIII. 

 THE PHYSIOLOGY OF SLEEP. 



The state of more or less complete unconsciousness which we 

 designate as sleep forms a part of the physiology of the brain which 

 naturally has attracted much attention, and the theoretical explana- 

 tions that have been advanced at one time or another are exceed- 

 ingly numerous. The same condition occurs in many, if not all, of 

 the other mammalia, and, indeed, in all living things there occur 

 periods of rest alternating with periods of activity. Whether these 

 periods of rest are essentially similar in nature to sleep in man 

 is a question in general physiology that can be solved only when 

 we know more of the chemistry of living matter. Within the human 

 body there are other tissues that exhibit periods of rest alternating 

 with periods of activity, the gland cells, for example. The secret- 

 ing cells of the pancreas have a period of activity in which the 

 destructive processes exceed the constructive, and a period of rest 

 in which these relations are reversed. We may compare this con- 

 dition in the gland cells with that in the brain. Sleep, from this 

 standpoint, is a period of comparative rest or inactivity, during 

 which the constructive or anabolic processes are in excess of the 

 disassimilatory changes. The period of sleep is a period of re- 

 cuperation, and doubtless all tissues have these alternating 

 phases. To explain sleep fundamentally, therefore, it would be 

 necessary to understand the chemical changes of anabolism and 

 catabolism, and an explanation of the sleep of the brain tissues 

 would doubtless explain the similar phenomenon in other tissues. 

 But what the physiologists desire first, and have attempted to 

 determine, is an explanation of why this condition comes on with 

 a certain periodical regularity, an explanation, in other words, of 

 the mechanism of sleep, the change or changes in the brain or the 

 body which reduce the metabolism of the brain tissue to such 

 an extent that it falls below the level necessary to cause conscious- 

 ness. 



Physiological Relations during Sleep. The central and most 

 important fact of sleep is the partial or complete loss of conscious- 

 ness, and this phenomenon may be referred directly to a lessened 

 metabolic activity in the brain tissue, presumably in the cortex 

 cerebri. During sleep the following changes have been recorded: 



238 



