SOMNAMBULISM, SLEEPLESSNESS, AND NIGHTMARE.
One of the characteristics of all living things is their periodicity. Thus plants have their period of activity, culminating in the produc tion of flower and fruit, and their period of in activity during which they appear quite without life and sap. Not only have they their annual periods of activity and inactivity, but also daily periods, their alternations being so largely dependent upon light and heat. Thus while ordinary flowers flourish mainly during the daytime, the cactus blooms and emits its perfume during the night. Animals also exhibit this periodic character. Most notable examples are the hybernating animals, who with the corning of winter's cold are sub ject to a remarkable diminution of their bodily processes, so that they pass the winter in a condition of unconsciousness, waste being re duced to the lowest, and the bodily function being just performed sufficiently to maintain the spark of life, which the return of heat re animates into a flame. Human beings also present many instances of a periodic character. The chief is undoubtedly exhibited by the heart. This organ might seem at first to be perpetually active during the whole of a person's lifetime; but it is not so. The movement of the heart is rhythmic, the rhythm consisting in regularly alternating periods of work and of rest. Thus, at the close of each contraction of the heart, signified by the beat against the chest wall, there is a pause, and the period of contraction is in the proportion of iths to that of the rest, which is ; the heart, that is to say, works 2ths of its time and rests iths. Similarly after every expiration from the lungs there is a pause before the next inspiration begins. These periods of rest are necessary for the continued vigour of the organs. The heart that was kept in such a state of excitement that it had little or no intervals between its contractions would be speedily exhausted, and would require to cease its work altogether. It is undoubtedly during the periods of repose that the energy expended in the immediately preceding activity is mainly renewed, and consequently, if, through want of the repose, the needed replenishment of energy did not take place, exhaustion would speedily follow. Now sleep is just another manifesta tion of this periodic character of living pro cesses. Sleep affords the interval during which nervous energy expended during the waking hours is renewed. In the waking state the
mind is being constantly appealed to. From all sides, by means specially of eye, ear, nose, and other sense organs, demands are made upon the attention of the brain, and during this waking state there cannot be a moment when the mind is not occupied somehow or other. All this means expenditure of force, and con sequent waste, which must be repaired. There therefore comes a time when this expenditure has reached a point when it is of advantage that it should cease, and the desire for sleep arises. The desire may, however, be fought, and the activity of the brain continued by de mand. This activity by demand means a greater than usual expenditure of nerve-force, a greater than usual degree, that is, of nervous exhaus tion. But it is obvious that the time will speedily come, be the stimulus what it may, when the cells that are the source of the energy can no longer respond to the demand, and sleep will supervene, no matter how resolutely it may be resisted, and no matter what objects are brought before the mind with a view to main tain its attention. Remarkable instances of this fact are given, such as those of gunner boys falling asleep during the height of an action, owing to the fatigue caused by their labours in carrying ammunition for the gunner. A case is also reported of a captain of a warship, en gaged in the last attack upon Rangoon, falling asleep, and remaining so for two hours beside one of the largest guns of his ship, the gun being served vigorously all the tune. Instead of sleep supervening, however, it often happens that, when sleep has been kept off by great mental effort or strain for a considerable time, a condition of irritability is set up which pre vents the access of sleep when it is at length desired. Business men, literary men, students, who night after night occupy themselves with business cares or studies far beyond the usual hour of retiring, are apt at length to produce a condition of extreme excitability and restless ness, which prevents them going to sleep with out delay when at last they do retire, and which prevents the sleep being either refreshing or restoring. This is properly an abnormal con dition, a condition upon the very borders of disease, if not actually disease. It indicates the revolt of the nervous centres to the treat ment they have received, a revolt which may pass into something very much worse if the cause of it be not speedily done away with.