SLEEP. 



685 



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 disturbance in 

 the slumbers of the other. The influence of 

 habitual attention is shown as much in the 

 effect produced by the cessation, as in that of 

 the occurrence, of sensory impressions. Thus 

 in the case of the naval officer aroused by the 

 suspension of the measured tread of the 

 watch over his head, the knowledge possessed 

 during the waking state that this suspension 

 is either an act of negligence which requires 

 notice, or indicates some unusual occurrence, 

 doubtless augments the effect which the dis- 

 continuance of the sound would of itself 

 produce. 



Putting aside the awakening influence of 

 external impressions, the period of natural 

 termination of the slumber is greatly influenced 

 by habit. Thus, many persons who are ac- 

 customed 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 pro- 

 found sleep at the precise time fixed upon. 

 In others, however, the desire to rise at a 

 particular hour only induces a state of rest- 

 lessness 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 

 oversleeps it after all, the system being worn 

 out by the need of repose. 



The Amount of sleep required by man is 

 affected by many conditions, especially age, 

 temperament, habit, and previous exhaustion; 

 so that no general rule can be laid down 

 upon the subject. The condition of the fetus 

 in utero may be regarded as one of continual 

 slumber ; the energy of the organic functions 

 being entirely directed to the building-up of 

 the organism, whilst the apparatus of animal 

 life is completely secluded from all the stimuli 

 which could arouse it into activity. On its 

 first entrance into the world, the infant con- 

 tinues 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 

 any external objects when its internal cravings 

 are satisfied; and even the eight months' 

 infant being considerably less alive to sensory 

 impressions, than one born at the full time. 

 During the whole period of infancy and child- 

 hood, it is necessary for the development of 

 the body that the constructive operations 

 should be more energetic than the destructive; 

 and, accordingly, we find that the period of 

 sleep, during which the former take place 

 without hindrance, is longer in proportion to 



the waking state, during which the latter ara 

 in play, than it is when full growth has been 

 obtained.* 



As age advances, the necessity for very 

 rapid nutrition gradually diminishes, in conse- 

 quence of the progressive approach to com- 

 plete development ; and when the adult 

 period has been attained, it is not requisite 

 that the constructive processes should do more 

 than balance the destructive. The amount of 

 sleep requisite for this purpose, therefore, 

 gradually diminishes, until it is reduced to (at 

 most) one-third of the cycle of twenty-four 

 hours. It is to be noticed that the sleep of 

 children and 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 waking state 

 decreases, a smaller amount of sleep suffices ; 

 or, if the slumber be protracted, it is usually 

 less deep and refreshing. It may be noticed, 

 however, that very old persons usually pass 

 a large proportion of their time in sleep, or 

 rather in dozing ; as if, in consequence of the 

 want of energy of their nutritive operations, 

 a very long period of repose is necessary 

 to repair the waste which takes place during 

 their short period of activity. It is stated -j- 

 that " the celebrated De Moivre slept twenty 

 hours out of the twenty-four ; and Thomas 

 Parr latterly slept away by far the greater 

 part of his existence." The repose of the 

 aged is most apt to take place immediately 

 after taking food ; while they solicit it in 

 vain at that period at which, during the 

 former years of their lives, they had been ac- 

 customed to enjoy it. 



The amount of sleep, again, is much af- 

 fected by temperament. It will generally be 

 found that a plethoric habit of body, sustained 

 by full diet, predisposes to sleep, provided 

 the digestive powers be in a vigorous con- 

 dition. Such persons frequently pass nine 

 or ten hours in slumber, and maintain that 

 they cannot be adequately refreshed by less. 

 On the other hand, thin wiry people, in whom 

 the " nervous" temperament predominates, 

 usually take comparatively little sleep, not- 

 withstanding the greater activity of their ner- 

 vous system when they are awake ; but their 

 slumber, while it lasts, is generally very deep. 

 Persons of " lymphatic" temperament, heavy 



* It is to be remembered, when we compare the 

 condition of the nutritive operations during the 

 period of growth, and after the complete development 

 of the organism, that it is not in the mere amount 

 of accretion that the difference consists. This would 

 be the case if the new matter were merely added to 

 the old, as in the formation of a new layer of wood 

 in an exogenous stem. The growth of the animal 

 fabric requires a continual new development of every 

 part of it, involving a constant change in its mate- 

 rials ; and thus we see that the amount of food 

 required by children, and the quantity of urea, and 

 other products of the disintegration of the tissues 

 set free in their excretions, bear a much larger 

 proportion to those of the adult, than would be 

 inferred from the relative bulk of the body at the 

 two periods, and from its rate of increase during the 

 former. 



f Macnish, op. cit. p. 37. 



