824 OF THE FUNCTIONS OF THE NERVOUS SYSTEM. 



the suspension of the measured tread of the watch over his head, the know- 

 ledge possessed, during the waking state, that this suspension is either an act of 

 negligence which requires notice, or indicates some unusual occurrence, doubt- 

 less augments the effect which the discontinuance of the sound would of 

 itself produce. It is not requisite, however, that the sound should be one 

 habitually attended to during the hours of watchfulness ; for it is sufficient if it 

 be one on which the attention has been fixed as that at which the slumberer is 

 to arouse himself. Thus the medical man, even in his first profound sleep after 

 a fatiguing day's work, is aroused by the first stroke of the clapper of his night- 

 bell ; and to those who are accustomed to rise every morning at the sound of 

 an alarm-clock, the frequency and regularity of the occurrence do not diminish, 

 but rather increase the readiness with which it produces its effect, provided that 

 the warning be promptly obeyed. On this usually depends the efficiency of the 

 awakening sound; if it be disregarded as a thing to which there is no occasion 

 to give heed, it very soon ceases to produce any effect, the entire peal not being 

 sufficient to awake the sleeper; whilst, on the other hand, the first stroke is 

 enough to break the repose of him who is impressed with the effectual desire of 

 profiting by the warning. And thus it may happen that, of two persons in the 

 same room, either shall be at once aroused by a sound which produces no dis- 

 turbance in the slumbers of the other. To this influence of previous impres- 

 sions, whether habitual, or but once forcibly made, we are also to refer the spon- 

 taneous termination of the state of sleep at particular times, without any sensorial 

 excitement from external impressions. Thus, many persons who are accustomed 

 to rise at a particular hour, wake regularly at that hour, whether they have 

 gone to rest early or late ; so that the act of spontaneously awakening is no 

 proof that the desirable amount of repose has been obtained. But what is more 

 remarkable is, that many individuals have the power of determining, at the 

 time of going to rest, the hour at which they shall rise, so as to awake from a 

 profound sleep at the precise time fixed upon. In others, however, the desire 

 to rise at a particular hour only induces a state of restlessness throughout the 

 night, destroying the soundness of the slumbers : the individual awakes many 

 times in the night, with the belief that the hour is past, and very possibly over- 

 sleeps it after all, the system being worn out by the need of repose. 



844. The Amount of Sleep required by Man is affected by so many condi- 

 tions, especially age, temperament, habit, and previous exhaustion, that no 

 general rule can be laid down'on the subject. The condition of the foetus in 

 utero may be regarded as one of continual slumber; the apparatus of animal life 

 being completely secluded from all stimuli which could arouse it into activity, 

 whilst the energy of the organic functions is entirely directed to the building up 

 of the fabric. On its first entrance into the world, the infant continues to pass 

 the greater part of its time in slumber; and this is particularly to be noticed in 

 cases of premature birth, the seven months' child seeming to awake only for the 

 purpose of receiving food, and giving but little heed to external objects, whilst 

 even the eight months' child is considerably less alive to sensory impressions 

 than one born at the full time. The excess of activity of the constructive over 

 the destructive operations, which characterizes the whole period of infancy, 

 childhood, and adolescence ( 129 131), requires that a larger proportion of 

 the diurnal cycle shall be passed in sleep (during which the former may be 

 carried on without hinderance), than is requisite when adult age has been at- 

 tained, the two sets of changes being then balanced ( 132) ; and the amount 

 of sleep to which the system shows itself disposed, gradually diminishes from 

 three-fourths to one-half, and from one-half to one-third, or even to one-quarter, 

 of the twenty-four hours. It is to be noticed that the sleep of children or 

 young persons is not only longer than that of adults, but is also more profound. 

 On the other hand, as age advances, and the bodily and mental activity of the 



