SLEEP. 647 



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 im- 

 portance 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. Eepose is as 

 necessary to the nutrition of the muscular system as proper exercise ; but re- 

 pose of the muscles relieves the fatigue due to exercise, without sleep. It 

 is true that after violent and prolonged exertion, there is frequently a desire 

 for sleep, but simple repose will often restore the muscular power. After the 

 most violent effort, 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 which involves an excessive expenditure 

 of the so-called nerve-force, sleep becomes an imperative necessity. If the 

 nervous system be 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, viz., those 

 concerned in circulation and respiration, are never completely arrested, sleep- 

 ing or waking, although they undergo certain modifications. 



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

 sleep is more important than in adult life or old age. The infant does little 

 but sleep, eat and digest. In adult life, under perfectly physiological condi- 

 tions, a person requires about eight hours of sleep ; some need less, but few 

 require more. In old age, unless after extraordinary exertion, less sleep is 

 required than in adult life. Each individual learns by experience how much 

 sleep is necessary for perfect health ; and there is nothing which more com- 

 pletely 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 which is well recognized 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 con- 

 tinued during sleep. In forced marches, regiments have been known to 

 sleep while walking ; men have slept soundly in the saddle ; persons will 

 sometimes sleep during the din of battle ; and other instances illustrating 

 the imperative demand for sleep after prolonged vigilance might be cited. 

 It is remarkable, also, how noises to which one has become accustomed may 

 fail to disturb natural rest. Those who have been long habituated to the 

 noise of a crowded city frequently find difficulty in sleeping in the stillness 

 of the country. Prolonged exposure to intense cold induces excessive som- 

 nolence, and if this be not resisted, the sleep passes into stupor, the power 



