432 POPULAR SCIENCE MONTHLY. 



THEOEIES OF SLEEP. 



By PERCY G. STILES, 



UNIVERSITY AND BELLEVUE HOSPITAL MEDICAL COLLEGE. 



"TT is not easy to define the condition of sleep in terms that will 

 -*- not admit of many exceptions. We readily recognize the states 

 of rest and activity, but where the element of consciousness must be 

 considered we are at once upon uncertain ground. If we think of 

 sleep as an unconscious state, sharply contrasted with waking, we do 

 well to limit our use of the word to the case of man and the most 

 intelligent animals. Sleep in this sense is only to be associated with 

 highly developed nervous systems and its final explanation is to be 

 sought in events taking place in the brain. 



Various writers upon the subject of sleep have turned their atten- 

 tion to quite different aspects of the matter. Some have undertaken 

 to show why there is the need of sleep and why the tendency to sleep 

 comes on at the close of each day. These writers have dealt with 

 general or systemic causes. Others have concerned themselves with 

 the cause of the unconscious — or dreaming — state in which the sleeper 

 lies. They have endeavored to suggest intimate and local causes. 

 Since the several theories are thus distinct in their application they 

 are not necessarily mutually exclusive. 



Broadly speaking we feel sure that the need of sleep follows from 

 general or local fatigue. During waking hours the decomposition 

 processes of the body doubtless rise above the life-long mean, and 

 sooner or later there must be a ceonpensatory fall below the average. 

 The adaptation of the race to alternating light and darkness has made 

 this rhythmic rise and fall to coincide with day and night — though 

 less rigidly under the artificial conditions of civilized life than in 

 more primitive times. ^ 



Fatigue at bottom is a chemical phenomenon, and so the theories 

 of the first class are chemical. When a muscle has been stimulated 

 until it exhibits the well-known signs of fatigue, there are two possible 

 inferences — either this means an exhaustion of fuel substances or an 

 accumulation of poisonous waste. Analogous views have been sup- 

 ported in regard to the chemical changes that lead to sleep. We have 

 had an exhaustion theory advanced by Pfiliger and an accumulation 

 theory offered by Preyer. 



