THE MYSTERY OF SLEEP
By OUR MEDICAL CORRESPONDENT
CONSIDERING that the problem of sleep has fascinated poets, philosophers, and natural observers since the beginning of history, it is curious how little is exactly known about the processes upon which it depends, and how rela- tively recent are the beginnings of such knowledge as we have. Such obvious facts as that, during sleep, there is a cessation of functioning of the sense organs and a complete relaxation of the voluntary muscles engaged in our waking activities have, of course, long been appreciated. It has also long been equally obvious that, although its rate is slightly lowered, the heart continues to beat normally during sleep, the lungs to expand and con- tract, and the processes of digestion to be maintained. In other words, it is principally the cortical or higher, or evolu- tionarily later-developed, regions of the brain, and the voluntary nervous system that subserves these, that are affected by sleep. But recent work has suggested that sleep itself is nevertheless primarily a function of the more primi- tive nervous system—or sympathetic system, as it is called— and the earlier portions of the brain.
This more primitive, sympathetic nervous system, which is structurally different from the voluntary nervous system that mainly deals with our intelligent, purposive activities, is responsible for the maintenance and co-ordination of our less conscious vital processes. It is also closely connected with what are known as the endocrine glands, such as the thyroid in the neck, the supra-renals (capping the kidneys), and what might perhaps be termed the master-gland, or pituitary, sunk and immensely carefully guarded in a bony receptacle in the base of the skull. These glands produce internal secretions of vital importance to growth, reproduc- tion and health in general, and are intimately associated with such emotions as anger and fear and their effects. The reactions due to these, although to a varying extent under the control of the higher nervous system and brain areas, are mainly conditioned by the sympathetic nervous system, which roughly speaking, maintains the body in a state of pre- paredness for their results and the commands communicated by the higher, later-developed, brain and nerve cells and fibres.
But there is a small portion of this primitive nervous system, known as the para-sympathetic, that has a somewhat different action. Its main functions are inhibitory. It can slow down the action of the heart and contract the pupil of the eye. It is, in fact, a sort of brake that, in times of stress or undue fatigue, helps to conserve valuable energy.
A few years ago, as the results of a series of investigations and experiments, Professor W. R. Hess, of Zurich, came to the tentative conclusion that it was this part of the sym- pathetic nervous system which was principally responsible for the actual process of sleep. He found that, by injecting a sub- stance which stimulated these inhibitory para-sympathetic nerves, he could induce a condition in animals that was in- distinguishable from normal sleep. He also found that he could obtain the same result by electrically stimulating certain deep-lying areas of the earlier brain, already known to be closely associated with these primitive nervous bundles.
The advance of modern brain surgery and cerebral photo- graphy has tended to produce supporting evidence in favour of this localisation of the sleep-engendering processes, since certain tumours in the same region have been found to be accompanied by changes in respect of sleep. It has also been suggested that there may possibly be a definite sleep- hormone or internal secretion ; and although this awaits con- firmation, certain experiments conducted by Zondek and Bier, of Berlin, seemed to show that the bromine contents of the pituitary gland were emptied during sleep into a portion of the earlier brain stem and that the gland refilled again before the act of waking.
It will thus be seen that definite appThaches would appear to have been made towards the solution of the problem of sleep from the purely physical and chemical aspects. But there are clearly also extremely important mental or psychical factors involved. Thus Pavlov found, in his patient and very well-known observations on the reactions of dogs to various groups of stimuli, that upon receiving a stimulus that led, as it were, to nothing in particular, a dog would on the first two or three occasions prick its ears and pay attention. If the stimulus was repeated, however, and if all competing stimuli were carefully excluded, the dog would cease to pay attention and ultimately go to sleep— interpreted by Pavlov as a process of deliberate inhibition that slowly spread itself until it had affected all the higher areas of the brain.
Similarly the withdrawal, as most people have noticed, of attention from a dull sermon or lecture in circumstances where escape is not possible, is often accompanied by somno- imce or actual sleep. Again, it is common knowledge that many people have the power to " compose " themselves to sleep and others to go to sleep when they " will," during an odd half-hour at a railway station or in the course of a journey. Further, while many drugs can induce sleep— itself indirect evidence, perhaps, of the existence of a definite chemical or tissue factor in the sleep process—probably the majority of cases of ordinary insomnia can be cured by self- discipline of a psychical or mental kind. The resolution to lie quietly in bed for the usual number of hours, to abstain from the temptation to get up, turn the lights on and read a book, will often in itself suffice to bring sleep in due course. The withdrawal to a room of one's own—thereby removing the perhaps unconscious envy of some other person known to be quietly sleeping, or even the elimination of a ticking clock, may and often does break the habit of insomnia.
The final solution of the mystery of sleep, should it ever be found, would thus perhaps throw more light on the full nature of man than that of most of our biological problems.
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