2i8 SCIENCE PROGRESS 



vigour of the cerebral circulation, and in many cases must be 

 regarded as directly due to it ; but as the heart is not in every 

 case of nocturnal mental activity correspondingly excited, 

 we are justified in regarding mental occupation as in itself a 

 sufficient cause of insomnia. 



While, then, there is a type of sleep referable to a definite 

 physiological condition — negative or positive — either the absence 

 of vigorous cerebral circulation, or of sensory stimulation, or 

 of mental activity, or the presence of fatigue-toxins — yet normal 

 somnolence is the result of the simultaneous co-operation of all 

 these factors in different degrees of relative intensity. 



We must not hold an exclusively vaso-motor theory of sleep, 

 nor an absence-of-sensation theory, nor an absence-of-mental- 

 activity theory, nor the presence-of-fatigue-toxins theory, but 

 must regard physiological sleep as a periodically recurring 

 phenomenon due usually to all of these four co-operant factors ; 

 and similarly, therefore, we must recognise the existence of an in- 

 somnia referable to each of the types of sleep — a circulatory 

 insomnia, a sensory insomnia, a psychic insomnia and a toxic 

 insomnia. 



