500 THE WONDER OF LIFE 



proportion of water, and this fact comtats the theories 

 that explain sleep by dehydration '. 



When the animal can no longer keep its eyes open, and 

 has become almost quite unresponsive, the frontal lobe 

 shows cellular disturbances. If it is no longer kept awake, 

 it plunges into deep sleep, from which it awakens com- 

 pletely refreshed. It is quite normal again, and the alter- 

 ations in the brain have disappeared. It may be that 

 these experiments are on the way to the discovery of an 

 alleviation of one of the most terrible of human ills — 

 insomnia. 



Claparede's view is that sleep is more than a passive 

 obedience to internal physiological necessities, it is an 

 active defensive instinct. Just as the bird migrates in 

 autumn before there is external coercion, so we go to sleep 

 before there is an overpowering need. The physiological 

 conditions, such as the fatigue-producing substances, puU 

 the trigger of an old-estabhshed sleep-instinct. They 

 serve to make us take for the time being a great interest 

 in sleep. If a greater interest should be aroused, sleepiness 

 disappears like magic ; the child who could hardly keep its 

 eyes open, does not want to go to bed at all when there is 

 sudden news of a great fire to be seen. When our interest 

 for the moment is greater in sleep than in anything else, 

 and that imphes inducing external and internal conditions 

 to which we have become habituated, then we are asleep 

 before we know it. 



It comes to this, that long ago, those animals got on best 

 which estabhshed a rhythm of work and rest, corresponding 

 on the whole to the periodicity of day and night, and later 

 on that some of their successors got on best which developed 



