554 NIGHT-SLEEP. 



It has been supposed by some that it is to habit that our tendency to 

 Cause of night- sleep at night is to be imputed. It is, however, more properly 

 sleep. to be attributed to the ordinary circumstances of our life the 



day being spent in muscular or mental exercise, since we can then see 

 to perform our duties, and this tax upon the system being necessarily 

 followed by a feeling of weariness. Those animals which seek their food 

 in the dark sleep by day. It is not, therefore, to any external physical 

 condition that we should impute our nocturnal sleep, but to the interior 

 condition of our system, though it is quite true that physical agents, 

 such as cold, and others that have been mentioned, will provoke a sensa- 

 tion of drowsiness. 



In sleep we require additional warmth, and this we obtain by instinct- 

 Increased ively using more clothing for the purpose of economizing the 

 warmth re- animal heat. The amount of caloric generated in the system 

 eep * is diminished through the cessation of muscular exercise, 

 and therefore reduction of decay. The same may be said, to a certain 

 extent, of the waste of the brain through its intellectual acts, and of the 

 nervous system generally. This diminished amount of interstitial death 

 corresponds with a diminished respiration, the hourly amount of oxygen 

 consumed exhibiting a decline. The negro, who is much more sensitive 

 than the white man to this decline of temperature, instinctively envelops 

 his head with clothing, so that the air may be warmed by its conta'et 

 therewith before it enters the respiratory organs. For the same reason, 

 he sleeps with his head toward the fire, while the white man sleeps with 

 his away. On similar principles we may account for the control which 

 food has over sleep, the one seeming, to a certain degree, to replace the 

 other. The French proverb says, "He who sleeps, dines," and this is 

 Uniformity of true ; for during sleep the waste of the system is reduced to 

 ed wiSTmii-*" a mm i m um, and the necessity for food correspondingly di- 

 formityoffood. minished. The quality of the food likewise exerts an influ- 

 ence on the length of sleep, for that which is of a nutritious kind, and 

 easily assimilated, can more speedily execute whatever repairs the sys- 

 tem may demand. It is probably owing to his variable diet that, even 

 in a state of perfect health, man is so variable a sleeper, and that ani- 

 mals, the nature of whose food is so constant, sleep with so much uni- 

 formity. 



By some it has been supposed that the amount of sleep required by 

 different animals is dependent upon the size of their brain; but if we 

 keep in view that the object of sleep is the repair of waste, and that this 

 is accomplished by the agency of the different mechanisms involved in 

 organic life, we can easily see that such a statement can not be true. Its 

 fallacy appears from common observation, apart from any physiological 

 considerations. The brain of a turtle or of a serpent is relatively small, 



