318 THE NERVOUS SYSTEM 



withdrawn from the brain. Much of the internal muscu- 

 lar and secretory mechanism becomes comparatively inac- 

 tive. The whole body is affected by the changes brought 

 about by sleep. We may describe the effects of sleep, 

 though we are not able to tell what sleep is, or what are 

 its causes. 



That sleep is essential to the life of all those animals 

 having a well-developed nervous system we know. Con- 

 tinued loss of sleep in man results in insanity and death 

 much sooner than death follows starvation. Regular 

 periods of sleep are needful, and nature provides for rest 

 to the brain by gradually reducing the amount of blood 

 circulating there when the accustomed hour for repose 

 draws near. Then, as we say, we become "sleepy." 



465. Necessary Amount of Sleep. Sleep, to be recupera- 

 tive, should continue uninterrupted for several hours. It 

 has been shown by experiments upon certain animals that 

 usually four hours or more are needed to reconstruct the 

 shrunken cells. Short " naps," even though the aggregate 

 be sufficient, do not restore and refresh the system as does 

 continuous sleep. If the nerve cells are forced to work 

 again before being fully restored, they do so under great 

 disadvantages. Growing children require more sleep 

 than adults, and a healthy, active child is not likely to 

 take more than is good for him. For adults who work 

 vigorously with mjiscles or brain, the old rule of eight 

 hours is a safe one, though many are satisfied with less. 

 Much depends upon habit and individual peculiarities. 

 The old usually need less sleep than others. The proc- 

 esses of growth are long since' completed, the activities 

 of life are less, vital processes go on more slowly, and 

 complete repair of the various tissues no longer takes 

 place. 



