680 



SLEEP. 



cycle in every case is fulfilled in twenty-four 

 hours ; and this uniformity in their recurrence 

 would seem to indicate either an entire and 

 invariable dependence on external agencies, or 

 else a periodical tendency to sleep, inherent 

 in the animal kingdom, and corresponding 

 with the cycle of day and night. The ex- 

 perience of the human species seems to be 

 decisive in favour of the latter view. 



There is, among all tribes of mankind, a 

 general uniformity in the periods of slumber 

 and activity, which is scarcely inferior to that 

 observable among the lower animals ; yet we 

 find reason to believe that this periodicity is 

 a law of our own organic constitution, for it 

 is quite certain that it cannot be seriously de- 

 parted from without injury to the system, 

 and that, even where light and warmth are 

 continuous through the whole range of the 

 twenty-four hours (as during the summer in 

 arctic regions), the same periodical desire for 

 sleep manifests itself, resistance to which is 

 prejudicial to the health. As Dr. Whewell 

 justly remarks* " No one can doubt that the 

 inclination to food and sleep is periodical, or 

 can maintain, with any plausibility, that the 

 period may be lengthened or shortened with- 

 out limit. We may be tolerably certain that 

 a constantly-recurring period of forty-eight 

 hours would be too long for one day of em- 

 ployment and one period of sleep, with our 

 present faculties ; and all whose bodies and 

 minds are tolerably active will probably agree, 

 that, independently of habit, a perpetual al- 

 ternation of eight hours up and four in bed 

 would employ the human powers less advan- 

 tageously and agreeably than an alternation of 

 sixteen and eight." We may remark, however, 

 that when the habit has been once acquired, 

 the shortening of the cycle is probably not so 

 injurious as its extension. We know by ex- 

 perience that the habitual attempt to sustain 

 an uninterrupted activity during more than 

 sixteen or eighteen hours at a time, is either 

 unsuccessful, or, if successful, is very wearing 

 to the system. On the other hand, the ex- 

 perience of seamen who kept " watch and 

 watch " during long voyages without any ob- 

 vious injury to their health, indicates that if 

 the due amount of sleep be obtained within 

 every twenty-four hours, the division of the 

 cycle is not attended with any prejudicial 

 effect. On the whole, we may conclude with 

 Dr. Whewell, that, " when we have sub- 

 tracted from the daily cycle of the employ- 

 ments of men and animals, that which is to be 

 set down to the account of habits acquired, 

 and that which is occasioned by extraneous 

 causes, there still remains a periodical cha- 

 racter, and a period of a certain length, which 

 coincides witn, or at any rate easily accom- 

 modates itself to, the duration of the earth's 

 revolution. 



Causes of Sleep. The most potent of all the 

 causes of sleep, which is capable of acting by 

 itself, when in sufficient intensity, in opposition 

 to the most powerful influences tending to the 

 continuance of wakefulness, is the condition 

 * Bridge-water Treatise, p. 40. 



of the nervous system induced by its pro- 

 tracted functional activity. Sleep may thus 

 come on in the midst of the roar of cannon, 

 and this not merely in persons accustomed 

 to the noise, but in those who have never 

 previously experienced it. Thus it is on record 

 that during the heat of the battle of the Nile, 

 some of the boys who were over-fatigued fell 

 asleep on the deck. We have known a 

 listener to an orchestral performance drop 

 off in slumber during the noisiest part of the 

 grand finale. Again, the continued demand 

 for muscular activity is not incompatible with 

 the access of sleep. During fatiguing 

 marches, as in the retreat to Corunna, it has 

 been repeatedly noticed that whole battalions 

 of infantry have slumbered whilst in motion ; 

 muleteers frequently sleep on their mules, 

 coachmen on their boxes, and post-boys on 

 their horses ; and factory children, before the 

 shortening of the hours of work, were often 

 known to fall asleep whilst attending to their 

 machines. Bodily pain, again, yields before 

 the imperative demand occasioned by the 

 continued exhaustion of the powers of the 

 sensorial centres. Of this the medical prac- 

 titioner has frequent illustrations. It is well 

 known, too, that the North American Indians, 

 when at the stake of torture, will go to sleep 

 on the least remission of agony, and will 

 slumber until the fire is applied to awaken 

 them. It is related that Damiens slept during 

 his protracted tortures upon the rack ; ami 

 that this having been prevented by the con- 

 stant renewal of fresh torments, he spoke of 

 the want of sleep, a little before the termi- 

 nation of his existence, as the most dreadful 

 of all the sufferings he had endured. That 

 the strongest voluntary determination to 

 remain awake is forced to give way to the 

 demand for sleep produced by the exhaustion 

 of nervous power, must be within the ex- 

 perience of every one. 



It does not appear to be of any consequence 

 whether this exhaustion is produced by the 

 active exercise of volition, emotion, reflection, 

 or simple sensation. In all alike the sen- 

 sorial centres must participate; by all alike, 

 therefore, must their nervous substance be 

 subjected to that disintegration which cannot 

 proceed beyond a certain point without either 

 being repaired by sleep, or producing a state 

 of exhaustion which becomes fatal. Never- 

 theless, we find that the involuntary con- 

 tinuance of mental activity is unfavourable to 

 access of sleep, so as to oppose the action of 

 other predisposing influences ; and such per- 

 sistence will be found to be especially difficult 

 to check in cases in which the feelings are 

 concerned. The activity of the purely intel- 

 lectual operations, which can be suspended at 

 any moment, provided the feelings be not 

 interested in their continuance, predisposes to 

 sleep instead of preventing it. But the desire 

 to work out a result, or to complete the survey 

 of a subject, is an emotional state which in- 

 duces restlessness, remaining active until it is 

 gratified. So, again, anxiety or distress is a 

 most frequent cause of wakefulness ; the ex- 



