762 POPULAR SCIENCE MONTHLY. 



when the store of force in our brain has been already considera- 

 bly drawn upon by the whole day's work. If no recovery by 

 sleep is enjoyed, or it is imperfect, the consequences will invaria- 

 bly make themselves evident the next day in a depression of men- 

 tal vigor as well as in a rise in the personal susceptibility to 

 fatigue. The rapidity with which one of the persons I experi- 

 mented upon could perform his tasks in addition sank about a 

 third after a night journey by railway with insufficient sleep. 

 Another experimenter could detect the effects of keeping himself 

 awake all night in a gradual decrease of vigor lasting through 

 four days. This observation was all the more surprising, because 

 the subject was not conscious of the long duration of the disturb- 

 ance, and was first made aware of it incidentally by the results 

 of continued measurements on the causes of the manifestations 

 of fatigue. 



These experiments admonish us to give special attention to 

 the question of sleep with men who work with their minds. 

 This is of more especial importance for the growing generation, 

 because the susceptibility to fatigue, and consequently the need 

 of sleep, are much greater in the youthful brain than in that of 

 adults. The average duration of sleep has been studied by Axel 

 Key in Swedish pupils of different ages. He found that it ranges 

 from nine hours in children ten years old down to seven hours in 

 pupils of eighteen years. Children ten years old were found who 

 slept only six, and some of seventeen or eighteen years who had 

 to satisfy themselves with four hours ! — a result which is really 

 astonishing. Axel Key is certainly right when he assumes that 

 the mass of Swedish school children of all ages are daily deprived 

 of one or two hours of their needed sleep, to say nothing of those 

 unfortunate ones who can sleep only half the time or less which 

 is required for their healthy mental and bodily development. 



The amounts of sleep required by different men are very va- 

 rious, for they are dependent on the deepness of the slumber. 

 There are persons who sleep so soundly that a surprisingly short 

 time spent in sleeping is enough for them. On the other hand, 

 we know that for many idiosyncrasies a length of sleep which is 

 quite enough for the average of men is much too short. 



Besides sleep, which limits the waste for a certain time and 

 favors the restoration of what has been consumed, we need for 

 the maintenance of our working strength the assimilation of food. 

 By means of food the substances are introduced to the tissues 

 which they require for their constant renewal. Sleep alone can 

 indeed retard for a long time the continued destruction of the 

 organs by the processes of life, as it does in the hibernation of 

 animals ; but there comes a point at last when only the introduc- 

 tion of fresh restorative matter can assure the continued main- 



