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Theories of Why We Sleep

There were no signs of genuine sleep; that is, relaxation of the limbs, closure of the eyes, and slow, regular respiration  were  absent.  Thus,  the interpretation of these motionless states in animals, as sleep, as had been done by other observers, was shown to be without sufficient foundation.

Experiments  on human subjects demonstrated that they fell asleep after listening for prolonged periods to monotonous auditory stimuli, but only if there was a concomitant state of muscular relaxation. That this sleep was genuine and not allied to hypnosis, was demonstrated by the fact, that some of these subjects were able to relate short dreams on awakening. Subjects also fell asleep without listening to monotonous stimuli if muscular relaxation could be induced. Muscular tension, however, even of one limb, under like conditions of listening to monotonous stimuli, entirely inhibited sleep. It was possible to measure the degree of muscular relaxation by a modification of an instrument called the capillary electrometer, both on subjects in whom sleep was experimentally produced and in already sleeping subjects after they had retired for the night. It was definitely shown by means of this instrument, that sleep and muscular relaxation were parallel phenomena, viz., in subjects gradually falling asleep, the muscular relaxation gradually diminished, while in subjects deeply asleep, the state of muscular relaxation was reduced to zero. With hypnotized subjects, however, the instrument showed no variation in muscular tension, the same as in the artificially produced motionless states in animals.

Analyzing my series of experiments, it was found that listening to a monotonous sound stimulus tended to produce a drowsy state and finally sleep. Sleep also took place when the element of monotony was not used, thus demonstrating  that  this  factor  was unnecessary for the production of sleep. A limitation of voluntary movements may thus produce sleep, but this limitation must be of the nature of a muscular relaxation and not a muscular tension. Further experiments with electrical currents demonstrated that sleep failed to result so long as the stimulus was felt. Only when the current was reduced to zero or when the lowest threshold of perception was reached, which for the subject was the same as zero, did sleep take place. Thus sleep was a reaction to stimuli and when, in conditions of muscular relaxation, stimuli ceased to pour into the brain and keep it active, sleep resulted. The dependence of sleep on muscular tonus is shown by the fact that when we “fight” against drowsiness we do so by voluntarily placing our muscles in a state of tension. When we allow ourselves to relax, sleep results. Sometimes great fatigue tends to keep one awake, because the fatigue symptoms are localized in the muscles. Sleep is an instinct, an inhibition of muscular tension.

Monotonous stimuli keep us awake by pouring themselves into the brain and keeping it active, and sleep can only take place if these stimuli are reduced to zero or to the threshold. Yawning before sleep is an effort to bring about muscular relaxation. Thus  muscular relaxation is a necessary condition for sleep in all the higher animals. In the lower organisms sleep was probably limited to motionless states of relaxation, which had all the characteristics of a simple, elementary instinct or tropism. Those organisms survived which possessed these relaxed reactions to their greatest extent, and from these sleep arose. All motionless states in animals are not sleep, however, neither do they resemble sleep, but are probably  a variety  of  hypnosis, demonstrated by my experiments.  as These experiments on the nature of sleep and hypnosis suggested several other directions to which inquiry might be directed— namely,

1.How did sleep evolve?

2.What is the biological necessity for sleep?

Although it has been noted that primitive, moving unicellular organisms, when observed for hours at a time, were unceasingly active and showed no motionless states, yet sleep must have arisen at some stage of evolution from these primitive organisms. Presumably those organisms survived which possessed these motionless states to their greatest extent, and from these motionless states could probably be traced the phylogenetic origin of sleep. In the higher animals, however, that is, in those possessing a complex nervous system, these motionless states, as demonstrated by my experiments, were not sleep but a form of cerebral inhibition, a genuine hypnosis. Furthermore, the animals experimented upon possessed genuine spontaneous sleep states, whereas the motionless states induced in them were artificial and experimental. In the lower organisms these motionless states are not intelligent reactions, but probably blind mechanisms, and we must therefore not allow the interpretation of such phenomena  to  lead us  into anthropomorphism. Neither can they be said to arise from fatigue, because such states may be observed in organisms which have not been subjected to stimuli that would lead to fatigue.

Lower organisms, however, are very sensitive to light, but whether this influence to light is a chemical or a mechanical phenomenon cannot be discussed at present. For instance, many motile forms collect in regions of a given light intensity, some orient themselves towards the source of light and others away from it into shadows or where the light is diminished. We are dealing here with a process variously termed heliotropism or phototaxis.1 These light re actions may be decidedly rhythmic in character and because they usually result from sudden changes in the intensity of light, they seem compulsory and mechanical. It has been found, for instance, that a sudden increase in the intensity of light will cause restlessness in earth worms and fresh water planarians. Diminution of the intensity of light inhibits these restless reactions and causes the creature to come to rest, or if such a creature goes from a light area to a dark one its activity becomes reduced to a minimum, it becomes motionless and seems to fall asleep. It seems probable that out of these periods of immobility and rest sleep arose. Light is a distance receptor and the activity of these organisms ceased when these particular receptors failed to throw its nerve elements into activity. The same mechanism probably takes place in the sleep of man and the higher animals from the inhibition of distance receptors. If sleep is an instinct, it was not so in the primitive organisms, but in these creatures it was a tropism, a mechanical or chemical necessity for repose under conditions where light was absent. From this tropism-like reaction, sleep arose, a veritable impulse of living matter to higher and higher rhythmic activities, motility on the one hand, with its freedom of action and the consequent development of the nervous system, periodic immobility on the other, in the effort to protect this nervous system from the pernicious effects of over-activity. Thus those organisms which showed these rhythmic reactions of immobility and repair were those which survived in the biological struggle for existence. Let us investigate these complex reactions to light more closely. Sometimes instead of attaining a definite axial position or orientation to the source of stimulation, the organism as a whole will move from light to shadow or vice versa.

Whether or not these reactions are adaptive or mere mechanical automatisms is one of the most important questions of comparative  psychology.  Probably  the phenomenon, at least in the more primitive organisms, is not psychic, the light in these cases acting as a mere directive stimulus. The fact that in brainless planarians can be demonstrated the same sensitiveness to light, but that the reaction time to arrive at immobility is longer, speaks in favor of the mechanistic hypothesis.

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