As might be readily inferred, even if we were lacking experimental evidence, fatigue interferes with the normal activities alike of body and mind. One of the earliest and most conspicuous effects may be observed by any one in the people about him--a decrease in the rapidity of physical action. The child depleted of nervous energy, for whatever reason, will usually be slower than his fellows in performing the various activities of home or school. If observed during gymnastic exercises it may be noticed that his execution of the various commands is delayed; in responding to signals he is behind his comrades whose nervous capital is not so largely spent. And what is here said of the child is, of course, equally true in principle of the adult; the effect of fatigue in his case will be revealed in less lively, vivacious, and vigorous conduct in the affairs of business or of society. Mosso,[35] Burgerstein,[36] Scripture,[37] Bryan,[38] and others have been able to confirm by scientific experiment what people have thus long been conscious of in a way--that cerebral fatigue renders one slower, more lethargic in his activities. It seems clear, to hazard an explanation, that when nerve cells become depleted up to the point of fatigue Nature designs that they should be released from service in order that repair may take place. This rhythm of action and repose seems to be common to all forms of life. The phenomenon of sleep is an expression of this principle, and is characterized by almost entire absence of activity. [Footnote 35: Pedagogical Seminary, vol. ii, pp. 20 _et seq._] [Footnote 36: Ibid., _op. cit._] [Footnote 37: The New Psychology, pp. 128-132.] [Footnote 38: The Development of Voluntary Motor Ability, p. 76.]