260 



PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



of the sleep is represented by the height of the ordinates. Accord- 

 ing 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 deepei 



STRENGTH OF STIMULUS 

 800 



HOURS QS 10 L$ 20 2.5 3.0 3.5 4.0 45 SO 5.5 &0 



70 75 7.8 



Fig. 113. 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 testa 

 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.) 



period up to the third hour. That this is the case is perhaps 

 sufficiently demonstrated by the experience of every one, but 

 Weygandt has attempted to prove the point by direct experi- 

 ments. He found that for simple mental acts, such as the ad- 

 dition of pairs of figures, a short sleep was as effective as a longer 

 one, but for more difficult mental work, such as memorizing 

 groups of ten figures, efficiency was distinctly improved in 

 proportion to 'the length of sleep. It is probable that the curve 

 of intensity of sleep varies somewhat with the individual and 

 also with surrounding conditions. That individual variations 

 occur is indicated by the results obtained by two other observers, 

 Monninghoff and Piesbergen,* who used the same general 

 method as was employed by Koklschiitter. The sleeper was 

 awakened by auditory stimuli produced by dropping a lead 

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



