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sleep theories and the function of sleep

Histological investigations on planaria and earth worms seem to indicate that the photosensitive elements are distributed over the body surface. That the reaction to light is a mechanical or a chemical response without the involvement of consciousness or perception, a mere mechanism, is demonstrated by the fact  that brainless organisms become motionless when the light intensity is suddenly reduced, or what amounts to the same thing, when shadows are suddenly thrown over the bodies of the creatures. These reactions to shadows seem to be defence reactions, because a shadow would naturally herald the approach of an enemy. Then the organism becomes motionless, a condition under which it would be less likely to be perceived.

Analogous conditions are sometimes found in the higher animals, namely, simulation of death, but here the defence reaction is intellectual and not mechanistic. Thus these latter reactions are in a general way adaptive and serve a purpose in not only protecting the creature from external influences, but likewise have a reparative action. Sleep, therefore, in these lower organisms seems a mere rest state, a negative heliotropic reaction, because of the poverty of the creature in receptor organs. As the animal evolved, as the spinal cord became a complicated reflex mechanism and the brain the dominant organ of consciousness, the various receptors became more numerous and complicated, and parallel with this there arose rhythmic states of activity alternating with rest or sleep.


It is well known that we cannot get along without sleep and so the important question arises—why is sleep biologically necessary? Genuine sleep only exists in organisms with a developed nervous system, and it has been shown that the motionless states in lowly organisms, when in shadows or in darkness, are not sleep. Sleep also seems to be due to a cessation of activity of the receptor organs and this in turn causes a diminished activity of the central nervous system. In sleep, the brain and spinal cord alone seem to be the seats of diminished activity, for the body metabolism during sleep does not differ much from that of the waking state. Sleep is an organic need, in the same way that hunger is an organic need. The effect of complete sleeplessness, as shown by experimental evidence, is to cause severe changes in the nerve cells. Therefore, the activity of the nerve cells furnishes the key to sleep. The Nissl bodies (or granules) of the nerve cells accumulate during repose and disappear in activity, particularly under conditions of fatigue. In the brains of chickens and dogs which have been suddenly killed during sleep there has been found an increase of the Nissl bodies.

This substance, therefore, accumulates in the nerve cells during their functional inactivity, when the sensory stimuli pour ing into these cells from without are greatly diminished. Normal nerve cells, or nerve cells in a state of rest, show these Nissl bodies with great clearness. It is only in the fatigued cell or the cell which has been poisoned by toxic substances or through the influence of increased temperature in fever, that these bodies are disintegrated and in many cases completely disappear, giving the cell a washed-out appearance (chromatolysis).

Therefore, sleep is a mechanism for the repair of nerve elements which have become disintegrated from the bombardment of stimuli received by the various surface receptors and receptor organs of the special senses. Those organisms which by reason of rest and immobility when they went into darkness or shadows, showed the greatest repair, were the very organisms which survived in the evolutionary struggle and sleep evolved out of these motionless states. This reparative power is absolute, for no matter how great the fatigue or long the insomnia, only a few hours of complete sleep are necessary as demonstrated by some exact experiments on the loss of sleep in man. In ordinary sleep, the eyelids are lowered, and a position is assumed by the sleeper which tends to a relaxation of all the voluntary muscles. Certain changes take place in the pulse and respiration, the blood-pressure falls, the muscles become relaxed, the threshold of consciousness becomes very low. The reflexes are diminished or may entirely disappear. The restorative and refreshing effect of natural sleep upon the tired nervous system is a fact well attested by everyday experience. A profound sleep is refreshing; a broken sleep, even in snatches that are profound, or lying in a half-sleeping state, such as frequently occurs in insomnia, fails to restore the fatigued organism. But even the pernicious effects of a complete insomnia are completely balanced by a few hours of profound sleep, as has been shown by certain experiments on the loss of sleep. Sleep rests and refreshes one because of the muscular immobility and relaxation during sleep, the internal organs become less active, the nervous system rests, there is a decided lowering of mental tension. In other words, during normal sleep, there is a distinct reparative action. What happens if the body is deprived of sleep? We will consider this question under two heads:—the complete loss of sleep from an experimental standpoint and the involuntary partial sleeplessness known as insomnia. It is well known that absolute loss of sleep has a very pernicious, sometimes even a fatal effect upon the organism. In man, however, even the severest types of insomnia complained of by patients who are sufferers from some form of nervous or mental disease, are never absolute sleeplessness. In China and during the Inquisition in Europe, forced deprivation of sleep was not only a form of torture, but also was used as a form of capital punishment. Manaceine’s experiments on young puppies showed that the animals suffered more from loss of sleep than from deprivation of food. When the animals were absolutely deprived of sleep from periods varying from 96 to 120 hours, the result was invariably fatal, even if sufficient food were given during this interval. She concludes from her experiments, that sleep is even more necessary to animals endowed with consciousness, than food.

In animals which have been starved to death, but few changes can be found in the brain, while in animals which died of enforced insomnia, the most profound and irreparable changes occurred, such as capillary hemorrhage and fatty alterations in the nerve cells. The experimental loss of sleep, as applied to man, was carried out in a very systematic manner by Professor Patrick and Dr. J.A.Gilbert, of the University of Iowa.1 This was the first time that such  experiments were carried out on man, previous investigations having been limited to dogs. The subjects were kept awake for about ninety hours and a series of psychological tests comprising reaction time, motor ability, memory, attention, etc., were made at six hour intervals. In one of the subjects, during the second night, hallucinations of sight developed; the air seemed full of colored particles which appeared like gnats and were in constant dancing motion. In all the subjects, memory became very defective and the power of attention was greatly lowered. After the experiments were finished, sleep brought about a complete restoration, in about one-sixth to one-third of the time of the enforced insomnia. The development of hallucinations in the above experiments is of interest. One of my cases of hysterical insomnia was constantly troubled by grimacing faces; another case of protracted sleeplessness  would  see a panorama of animals just as he dozed off, at other times he would hear a voice constantly repeating “Let down the jib.” All these sense deceptions occurred in the half-sleeping condition, never when the subject was fully awake or fully asleep. Ordinary insomnia is a very common complaint. One of the most common causes is physical pain. It also occurs in many forms of nervous diseases, particularly neurasthenia. In this insomnia of neurasthenia, the subject is frequently in a half-waking and half-sleeping condition, with a hazy state of consciousness and limitation of muscular activity. Or sleep may be secured in snatches, but the slightest noise awakens the sleeper.

Therefore, in spite of their statements, these individuals never suffer from complete insomnia; they sleep more than they realize. Extreme physical exhaustion alone may produce insomnia, a proof that sleep is not absolutely dependent on exhaustion of the nerve centres. Sleeplessness may also be due to an emotional shock, as in certain cases of hysterical insomnia. For instance, a patient became greatly frightened by an insane woman entering her store and throwing an entire box of lighted matches among some paper. The patient immediately became greatly agitated, began to dream of the episode at night, and one week later, an insomnia developed, which continued for five years, up to the time she came under observation.

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