178 THK XKUVOrS SYSTKM AM) ITS ( '< ).\SK1(\ ATH >\ 



falling, two very common impressions. Hearing is the 

 l:it.--t means of communication between the world without 

 :ni(l the isolated mind. It is likewise the first channel to 

 ho reopened as one is roused or wakes gradually from 

 sleep. 



The depth of sleep at different times in the eouise of the 

 night has often been tested. In the trials most frequently 

 referred to, the measurement was based on the loudness of 

 the sound necessary to awaken the sleeper. All observers 

 have agreed that the deepest sleep, as indicated by such 

 methods, comes quite early, probably within the second 

 hour, if not the first. \Ye know that if we are awakened 

 within this period the exix-rience is one of violent subject- 

 ive and bodily disturbance. The nervous shock is such 

 as to make it hard to compose one's self again. It is a very 

 different matter to be aroused from the later and shallower 

 sleep; then the response to a call is easily obtained, but, 

 unhappily, it is correspondingly easy to sink back to 

 sleep. 



After the first deep sleep there is a shoaling which is 

 about as rapid as the previous subsidence 1 into the depths. 

 Within three hours of the beginning a condition is reached 

 in which there is marked sensitiveness to stimulation. 

 Opinions differ as regards the later behavior of the nervoii- 

 sy<tem. According to one set of data there is a gradual 

 and progressive reduction of the depth of sleep during the 

 three 1 or four additional hours of its continuance. Others 

 have noted a second deepening toward morning. 'This 

 la-t description appeal's to fit in with a great deal of in- 

 dividual experience; a wakeful time may be passed 

 through about ^ o'clock, and it may be harder to get up 

 two hours later than it would have 1 bevn at dawn. A 

 < lermaii \\ riter, accepting the double-peaked curve 1 a> nor- 

 mal, has referred it to the impression made upon the 

 centers when the infant woke once each night to be fe-el. 



Little children who no longer wake al this time are apt to 



show a restle-s interval be'tween two periods of heavy 



slumber. 



