684 



SLEEP. 



vigour, may generally be traced to some sen- 

 sory impression of a trivial nature, such as the 

 striking of a clock, which would have pro- 

 duced no effect at a previous time. Some 

 persons, however, always sleep so heavily, 

 that they require a strong impression to 

 arouse them, even when they have had an 

 ample allowance of repose. It is through the 

 hearing and the touch that the awakening im- 

 pressions are ordinarily conveyed ; but either 

 of the other senses may serve as their channel. 

 Thus, although the closure of the eyelids 

 destroys the acuteness of the perception of 

 light, the eyelids are sufficiently transparent 

 to allow of an impression being made by a 

 light of moderate intensity ; so that those who 

 sleep in a room whose window has an eastern 

 aspect, and is not furnished with sufficient 

 means of excluding the sun's rays, are liable 

 to be aroused by their ingress some time 

 before the natural amount of repose has been 

 taken. So, again, the sleeper may be awakened 

 by unusual odours ; thus the inmates of a 

 burning house are sometimes first aroused by 

 the smell of fire. The introduction of sub- 

 stances possessing a strong taste into the 

 mouth, will also usually put an end to the 

 state of slumber ; but when the slumber is 

 very profound, such substances may be re- 

 ceived, and even swallowed, without the 

 sleeper being thereby awakened. 



The variety of modes in which the oper- 

 ation of sensory impressions on the sleeper is 

 modified by the previous habitual state of 

 mind, is one of the most remarkable points of 

 the whole subject. The general rule is, that 

 habitual impressions of any kind have much 

 less effect in arousing the slumberer, than 

 those of a new or unaccustomed character. 

 An amusing instance of this kind has been 

 related to the author, which, even if not 

 literally true, serves extremely well as an 

 illustration of what is unquestionably the or- 

 dinary fact. A gentleman who had taken his 

 passage on board a ship of war, was aroused 

 on the first morning by the report of the 

 morning gun, which chanced to be fired just 

 above his berth ; the shock was so violent, 

 as to cause him to jump out of bed. On the 

 second morning he was again awoke, but this 

 time he merely started and sat up in bed ; on 

 the third morning the report had simply the 

 effect of causing him to open his eyes for a 

 moment, and turn in his bed : on the fourth 

 morning it ceased to affect him at all, and his 

 slumbers continued to be undisturbed by the 

 report as long as he remained on board. It 

 often happens that sleep is terminated by the 

 cessation of an accustomed sound, especially 

 if this be one whose monotony or continuous 

 repetition had been the original inducement 

 to repose. Thus, a person who has been read 

 or preached to sleep, will awake, if his slum- 

 ber be not very profound, on the cessation of 

 the voice ; and a naval officer, sleeping be- 

 neath the measured tread of the watch on 

 deck, will awake if that tread be suspended. 

 In this latter case, the influence of the simple 

 cessation of the impression will be augmented 



by the circumstance next to be alluded to, 

 which has received too little attention from 

 writers on this subject, but which is of pe- 

 culiar interest both in a physiological and 

 psychological point of view, and is practi- 

 cally familiar to almost every one. 



This is, that the influence of sensory 

 impressions is greatly modified by our habitual 

 state of mind in regard to them. Thus, if we 

 are accustomed to attend to these impressions, 

 and our perception of them is thus increased 

 in acuteness, we are much more easily aroused 

 by them than by others which are in them- 

 selves much stronger, but of which we have 

 been accustomed to entertain an utter dis- 

 regard. Thus, most sleepers are aroused by 

 the sound of their own names uttered in a 

 low tone, when it requires a much louder 

 sound of a different description to produce 

 any manifestation of consciousness. The same 

 thing is seen in comatose states ; a patient 

 being often capable of being momentarily 

 aroused by shouting his name into his ear, 

 when no other sound produces the least 

 effect. The following circumstance, commu- 

 nicated to the author by a naval officer of 

 high rank, is a most apposite illustration of 

 this principle. When a young man, he was 

 serving as signal-lieutenant under Lord Hood, 

 at the time when the French fleet was con- 

 fined in Toulon harbour ; and being desirous 

 of obtaining the favourable notice of his com- 

 mander, he devoted himself to his duty 

 that of watching for signals made by the look- 

 out frigates with the greatest energy anil 

 perseverance, often remaining on deck nine- 

 teen hours out of the twenty-four, with his 

 attention constantly directed towards this one 

 object. During the few hours which he 

 spent in repose, his sleep was so profound, 

 that no noise of an ordinary kind, however 

 loud, would awake him ; and it used to be a 

 favourite amusement with his comrades, to 

 try various experiments devised to test the 

 soundness of his sleep. But if the word 

 " signal " was even whispered in his ear, he 

 was instantly aroused, and fit for immediate 

 duty. 



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 alarum-clock, the fre- 

 quency and regularity of the occurrence do 

 not diminish, but rather increase, the readi- 

 ness with which it produces its effect, pro- 

 vided 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 



