OF SLEEP AND SOMNAMBULISM. 715 



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, how- 

 ever, that very old persons usually pass a large proportion of their time in 

 sleep, or rather in a sort of heavy doze, especially after meals ; as if, in con- 

 sequence 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. In regard to the influence of temperament, it 

 may be remarked that a plethoric habit of body, sustained by full diet, 

 usually predisposes to sleep, provided that the digestive powers be in a 

 vigorous condition ; persons of this constitution 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" temper- 

 ament predominates, usually take comparatively little sleep, notwithstanding 

 the greater activity of their nervous system when they are awake ; but their 

 slumber, while it lasts, is generally very deep. Persons of "lymphatic" tem- 

 perament, heavy passionless people, who may be said to live very slowly, 

 are usually great sleepers; but this is rather because, through the dulness of 

 their perceptions, they are less easily kept awake by seusorial or mental 

 excitement, than because they really require a prolonged cessation of activity. 

 As they are half asleep during the waking state, so it would appear that the 

 constructive operations must be far from active while they are asleep, so 

 little do they seem restored by the repose. The amount of sleep, cceleris 

 paribus, required by individuals, is very greatly influenced by habit; and, 

 contrary to what we might anticipate, we find that the briefest, sleepers have 

 usually been men of the greatest mental activity. Thus Frederick the Great 

 and John Hunter are said to have only required five hours' sleep out of the 

 twenty-four; and General Elliott, celebrated for his defence of Gibraltar, is 

 recorded not to have slept more than four hours out of the twenty-four. It 

 may be doubted whether it would be possible for any one to sustain a life of 

 vigorous exertion upon a smaller allowance than this ; and the general fact 

 is, that from six to eight hours of repose, out of every twenty-four, are re- 

 quired to keep the system of an adult in a state of healthful activity. The 

 influence of habit may be brought to bear upon the protraction, as well as on 

 the abbreviation, of the usual period. Thus Quin, the celebrated actor, could 

 slumber for twenty-four hours successively; and Dr. Reid, the metaphysician, 

 could take as much food, and afterwards as much sleep, as were sufficient 

 for two days. It is needless to dwell upon the obvious fact, that other 

 things being equal, the amount of sleep required by man is proportional to 

 the amount of mental exertion put forth during the waking hours; since this 

 is an obvious result of what has been laid down as the cause of the demand 

 for sleep. It may be remarked, however, that we must not measure the 

 amount of sleep by its duration alone, since its intensity is a matter of equal 

 importance. The light slumber which is disturbed by the slightest sounds, 

 cannot be as renovating as the profound sopor of those whom no ordinary 

 noise will awake. 



582. There are certain states of the Encephalic centres, in which there is an 

 entire absence of Sleep ; and this may continue for many days, or even for 

 weeks or months. Insomnia is, for instance, one of the characteristics of 

 acute Mania, and may also exist in various forms of Monomania; it is 

 usually, also, one of the symptoms of incipient meniugeal inflammation ; 

 and it may constitute a specific disease in itself. In all these cases, how- 

 ever, the preponderance of the destructive processes over the constructive 

 manifests itself, sooner or later, in the exhaustion of the mental and bodily 



