SLEEP. 447 



fileep. After prolonged and severe mental exertion, how- 

 ever, 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 is the only physi- 

 ological means of complete restoration ; but the two most 

 important muscular acts ; viz., those concerned in circulation 

 and respiration, are never completely arrested, sleeping or 

 waking, though they undergo certain modifications. 



In infancy and youth, when the organism is in process 

 of development, sleep is more necessary than in adult life or 

 old age. The infant does little but sleep, eat, and digest. 

 In adult life, under perfectly physiological conditions, we 

 require about eight hours of sleep ; some persons need less, 

 but very few require more. In old age, unless after extraor- 

 dinary 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 

 completely incapacitates one for mental or muscular effort, 

 especially the former, than loss of rest. 



Sleeplessness is one of the most important of the predis- 

 posing causes of certain forms of brain-disease, a fact which 

 is well recognized by practical physicians. One of the most 

 refined and exquisite methods of torture is long-continued 

 deprivation of sleep ; and persons have been known to sleep 

 when subjected to acutely painful impressions. Severe mus- 

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

 marches, regiments have been known to sleep while walking ; 

 men have slept soundly in the saddle ; persons will some- 

 times sleep during the din of battle ; and other instances 

 illustrating the imperative demand for sleep after prolonged 

 vigilance might be cited. 1 It is remarkable, also, how noises 



1 For a number of curious and interesting examples of sleep under the most 

 unfavorable circumstances, the reader is referred to the admirable work of Dr 

 Hammond (Sleep and its Derangements, Philadelphia, 1869, p. 14, et seq.). 



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