72 SLEEP. 



more than any other organ, cannot go on working 

 continuously without any time for restoration, 

 and since the <lay is tlie easiest period for men 

 and animals to see in as they go about their vari- 

 ous avocations, it is natural enough that the niglit, 

 "wlien no man can work," should be assigned as 

 the proper season for rest and repair. Almost 

 every kind of higher creature has accordiiigly 

 adapted itself to this obvious necessity. And so 

 ingrained therefore lias this habit become of passing 

 a long daily period in repose of the brain and 

 nerves that even those animals which have taken 

 to nocturnal habits, to escape their enemies or to 

 Becuie their p''ey, merely reverse the ordinary rule, 

 sleeping in the daytime and waking by night. In 

 / fact, though some very simi)le and lowly things 

 ! can do without sleep almost altogether, because 

 they only go catching f(jod and digesting it, 

 without eyes or thinking apparatus, no animals as 

 high in the scale as even fish or reptiles could 

 possibly get on without this needful period of rest 

 and slumber. And the higher tlie type of life the 

 greater the necessity for sleep. People who lead 

 very healthy out-of-door lives, working hard with 

 their limbs and muscles, and eating a sufficiency 

 of good plain food, can do with comjjaratively few 

 hours in bed. What the}'^ need to repair are 

 mainly the muscles and the nerves supplied to 

 them, as well as the eyes, the ears, audi, the sini- 



