242 USE AND CARE OF THE NERVOUS SYSTEM 



Although during sleep the lower nerve centres, espe- 

 cially those of the sympathetic system, are active, the 

 higher centres of the cerebrum are largely at rest. They 

 are both avoiding exhaustion by further work and re- 

 covering from the work already done by storing up a 

 fresh supply of food and eliminating waste products. 

 Sleep is thus a period of relaxation from outside work, 

 of cleaning up and setting to rights of each cell's house- 

 hold, and of laying in fresh stores of fuel for the mor- 

 row's demands. To deprive the nerve cells of sleep is 

 therefore not only to prevent each cell from recovering 

 its normal amount of strength, but to make it grow 

 weaker and weaker from overwork. To such abuse 

 there can be but one outcome. Inevitably the nervous 

 system must grow gradually less and less efficient until 

 finally it breaks down. 



Amount of sleep. Although sleep is required to keep 

 the nervous system in order, yet it has been found that 

 too much sleep is almost as bad as too little, since it 

 results in the nervous system becoming sluggish and 

 inefficient through too little exercise. The cells seem 

 to accumulate an oversupply of food materials and 

 thus become so clogged up that they work much less 

 quickly and easily. The amount of sleep which each 

 person needs depends less upon personal peculiarities 

 than is generally thought. Rather does it depend upon 

 how old a person is and how hard he works. In early 

 infancy when the brain and other tissues of the body 

 are least developed and are growing most rapidly, the 

 baby spends a very large part of the twenty-four hours 

 in sleep. Gradually the amount of sleep taken grows 

 less, until at the end of a year the child requires but 

 thirteen or fourteen hours. ' This amount is further 

 diminished until at ten years of age eleven hours are 



