RELATIONS OF INSOMNIA TO TYPES OF SLEEP 217 



postilions used to fall asleep in the saddle and yet maintain 

 their position, and more than once Holbein, the cross-Channel 

 swimmer, has been found swimming asleep. All this means that 

 sleep induced by extreme fatigue can supervene even although 

 there be present the most penetrating sensory stimulation ; the 

 synaptic resistance has been so raised that the impulses from 

 even violently stimulated sense-organs are prevented from 

 arriving at the sensory centres. 



The fourth factor productive of sleep is the absence of mental 

 activity in every form — the absence of strong emotion, of worry, 

 of intellectual problems — everything that can " keep the mind 

 awake." Nothing is more familiar than the fact that intellectual 

 occupation, especially if with distinct emotional colouring, can 

 effectually banish sleep — a psychic insomnia. 



Doubtless mental exaltation is accompanied by increased 



