THE PHYSIOLOGY OF SLEEP. 



243 



of the sleep is represented by the height of the ordinates. According 

 to this curve, the greatest intensity is reached about an hour after 

 the beginning, and from the second to the third hour onward the 

 depth of sleep is very slight. The activities of the brain lie just 

 below the threshold of consciousness. It appears also from this 

 curve that the recuperative effect of sleep is not proportional to 

 its intensity. The long period from the third to the eighth hour, 

 in which the depth of sleep is so slight is presumably as important 

 in restoring the brain to its normal waking irritability as the deeper 

 period up to the third hour. It is probable that the curve of in- 

 tensity of sleep varies somewhat with the individual and also with 

 surrounding conditions. That individual variations occur is indi- 



STRE1NGTH OF STIMULUS 

 800 



100 



HOURS 0,5 UO L5 Z.O 2.5 3.0 3.5 4.0 45 5.0 5-5 6.0 6& 7.0 7.5 7.8 



Fig. 106. Curve illustrating the strength of an auditory stimulus (a ball falling from 

 a height) necessary to awaken a sleeping pers9n. The hours marked below. The tests 

 were made at half-hour intervals. The curve indicates that the distance through which 

 it was necessary to drop the ball increased during the first hour, and then diminished, at 

 first very rapidly, then slowly. (Kohlschiitter.) 



cated by the results obtained by two other observers, Monninghoff 

 and Piesbergen,* who used the same general method as was em- 

 ployed by Kohlschutter. The sleeper was awakened by auditory 

 stimuli produced by dropping a lead ball from varying heights upon 

 a lead plate. Only two experiments were made each night, and 

 the curves constructed represent, therefore, composites from several 

 periods of sleep. One of the curves obtained is represented in 

 Fig. 107. According to this curve, the maximum intensity is 

 reached between the first and second hours, and between the fourth 

 and the fifth hour there is a second slight increase in intensity, 

 giving a second maximum in the curve. This latter feature of a 

 * Monninghoff and Piesbergen, "Zeitschrift f. Biologic," 19, 1, 1883. 



