LENGTH OF THE DAY. 39 



Those animals which are nocturnal feeders are 

 diurnal sleepers, while those which are crepuscu- 

 lar, sleep partly in the night and partly in the 

 day ; but in all, the complete period of these func- 

 tions is twenty-four hours. Man, in like manner, 

 in all nations and ages, takes his principal rest 

 once in twenty-four hours ; and the regularity of 

 this practice seems most suitable to his health, 

 though the duration of the time allotted to repose 

 is extremely different in different cases. So far 

 as we can judge, this period is of a length benefi- 

 cial to the human frame, independently of the 

 effect of external agents. In the voyages recently 

 made into high northern latitudes, where the sun 

 did not rise for three months, the crews of the 

 ships were made to adhere, with the utmost punc- 

 tuality to the habit of retiring to rest at nine, and 

 rising a quarter before six ; and they enjoyed, 

 under circumstances apparently the most trying, 

 a state of salubrity quite remarkable. This 

 shows, that according to the common constitution 

 of such men, the cycle of twenty-four hours is 

 very commodious, though not imposed on them 

 by external circumstances. 



The hours of food and repose are capable of 

 such wide modifications in animals, and above all 

 in man, by the influence of external stimulants 

 and internal emotions, that it is not easy to dis- 

 tinguish what portion of the tendency to such 

 alternations depends on original constitution. Yet 



