MENTAL FUNCTION 



423 



during sleep, with the result that the eyeballs turn somewhat inward and 

 a good deal upward. General complete relaxation of the voluntary 

 muscles on retiring is an important habit to acquire, for only so are they 

 free of tonus, and this, as we have seen, is a condition of partial activity. 

 The unique position taken by the cadaver while lying flat is the type and 

 limit of this sort of muscular relaxation. 



Sleep varies not a little in its soundness at different times during the 

 night. It has been found (by comparing the intensities of sounds neces- 

 sary to awaken the sleeper) that the depth of sleep slowly increases (Mb'n- 

 ninghoff and Piesbergen) for 1.25 hours, then deepens very rapidly (see 

 curve, Fig. 243) for half-an-hour, and becomes speedily lighter during 

 the next half-hour. From this time on until the middle of the fifth hour 

 sleep very gradually becomes lighter, deepens slightly for another hour, 

 and then becomes slowly but progressively less deep until awakening 

 occurs. Thus a person retiring at ten o'clock is most profoundly asleep 



FIG. 243 



% 1 HR. 2 3 5 



Graphic representation of the varying depth of a natural sleep lasting about six hours. 

 Light for 1.25 hours, it rapidly deepens during the next 0.5 hour; becomes as rapidly lighter 

 for the next 0.5 hour; then more slowly for 2.25 hours; deepens slightly for 1 hour more; then 

 becomes slowly lighter until some slight stimulus is enough to recall the person to the waking 

 consciousness. (Altered from Piesbergen.) 



at quarter before twelve, and after the second hour the sleep is compara- 

 tively light and easily disturbable. Children sleep the most soundly, 

 but men more soundly than women. 



The mental relations of sleep are difficult to describe with certainty. 

 We think of sleep-consciousness as continuous but as lowered in intensity 

 and, which is more important, as separated in some manner from the 

 waking-consciousness. In abnormal cases one waking consciousness is 

 similarly shut off by forgetfulness from another, this amnesia giving us 

 the phenomena we call double or multiple personality. Aside from 

 dreams so vivid probably as to partially awaken us, we do not remember 

 the mental events experienced in sleep. Yet there is excellent and wide- 

 spread evidence that consciousness persists continually, for the more 

 suddenly a person is awakened the more certain he is to find himself 

 in the midst of conscious experiences of some sort. This is not to be 

 accounted for on any supposition other than that consciousness persists 



