618 NERVOUS SYSTEM 



SLEEP 



When it is remembered that about one-third of each day is passed in 

 sleep, and that at this time, voluntary motion, sensation, the special 

 senses and various of the functions of the organism are greatly modified, 

 the importance of a physiological study of this condition is sufficiently 

 apparent. The subject of sleep is most appropriately considered in 

 connection with the nervous system, for the reason that the most 

 important modifications in function are observed in the cerebro-spinal 

 axis and nerves. Repose is as necessary to the nutrition of the mus- 

 cular system as proper exercise ; but repose of the muscles relieves the 

 fatigue due to exercise, without sleep. It is true that after violent and 

 prolonged exertion there frequently is a desire for sleep, but simple 

 repose often will restore the muscular power. After the most violent 

 exertions, a renewal of muscular vigor is most easily and completely 

 effected by rest without sleep, a fact familiar to all who are accustomed 

 to athletic exercises. After prolonged and severe mental exertion, 

 however, or after long-continued muscular effort involving excessive 

 expenditure of the so-called nerve-force, sleep becomes an imperative 

 necessity. If the nervous system is not abnormally excited by effort, 

 sleep follows moderate exertion as a natural consequence, and it is the 

 only physiological means of complete restoration ; but the two most 

 important muscular acts, namely, those concerned in circulation and 

 respiration, are not completely arrested, sleeping or walking, although 

 they undergo certain modifications. 



In infancy and youth, when the organism is in process of develop- 

 ment, sleep is more important than in adult life or in old age. The 

 infant does little but sleep, eat and digest. In adult life, under physi- 

 ological conditions, a person requires about eight hours of sleep ; 

 some need less, but few require more. In old age, unless after extraor- 

 dinary exertion, less sleep is required than in adult life. Each indi- 

 vidual learns by experience how much sleep is necessary in perfect 

 health ; and there is nothing which more completely incapacitates one 

 for mental or muscular effort, especially the former, than loss of natural 

 rest. 



Sleeplessness is one of the most important of the predisposing 

 causes of certain forms of brain-disease, a fact that is well recog- 

 nized by practical physicians. One of the most severe methods of 

 torture is long-continued deprivation of sleep ; and persons have been 

 known to sleep when subjected to acutely painful impressions. Severe 

 muscular effort, even, may be continued during sleep. In forced 

 marches, regiments have been known to sleep while walking ; men 



