In an effort to explain the mechanism by which sleep is brought about, many theories have been offered, the best known of which are the so-called physiological, chemical, pathological, and histological theories.

The physiological theories would account for sleep by an alteration in the blood supply of the brain, due to the action of some agent or agents which causes a relaxation of the vaso-motor centre, the latter being concerned with the regulation of the size of the blood vessels. While it is true that sleep is favoured by cerebral anemia, and that the sleeping state is generally accompanied by a diminished supply of blood to the brain, it has been demonstrated that sleep can take place when cerebral hyperemia is present; also, that at various periods of sleep there occurs a physiological increase in the supply of blood to the brain without awakening being produced. For these and other reasons the vaso-motor theory has not received general acceptance.

Among other theories classified among the physiological are those which ascribe sleep to changes in the secretions of various ductless glands. At one time interest was centred in the thyroid and the hypophysis. The thyroid is a small gland situated just below, and on either side of the thyroid cartilage of the larynx, the latter receiving its popular name of Adam's Apple from the fable that a portion of the forbidden apple became lodged in Adam's throat in that situation. It is this gland which is enlarged in goitre. If the thyroid were essential for sleep we might reasonably expect that per sons deprived of it congenitally, by disease, or operation, would be victims of sleeplessness. We find, however, that in such cases lethargy is the rule; when the gland is enlarged disturbed sleep is common.

As a result of studies of hibernating animals, as well as clinical studies of individuals suffering from abnormalities of the hypophysis, various observers were led to conclude that a diminished secretion of this gland caused sleep. The hypophysis is another small gland, situated below the brain in the middle fossa of the skull. If this gland shows a diminished secretion during sleep, ordinary or prolonged, we should remember that sleep is accompanied by a lessened activity of all the physical and mental structures; the diminished secretion, may, therefore, be a result of sleep and not a cause of it.

The chemical theories, which may also be considered typical of the pathological, trace sleep to the narcotic action of various toxic products resulting from tissue work. We have no good reason to believe sleep a pathologic process; moreover, if this theory were true, we might expect that the lazy person, who has little tissue breakdown, would be an insomniac; experience teaches that, as a rule, it is the hard worker who suffers from sleeplessness.

The histological theories would explain sleep by various changes in the nerve structures. Thus, some have held to the view that sleep is brought about by a retraction of the nerve endings, due to chemical agents or even auto-suggestion, which prevents the nervous units from communicating with one another. This idea is not satisfactory because, in addition to other objections, the experimental evidence offered to uphold it was derived from studies of narcotized animals, and between narcotic sleep and natural sleep there are many and important differences. However, in humans we find some evidence to support it in the fact that the tremors of the paralysis agitans patient disappear in sleep.

The theories which have received most favourable consideration are those which attempt to solve the problem from a biologic or psychologic viewpoint. Claparede, in 1904, published his biologic theory which, while it had no experimental evidence to support it, excited much favourable attention. He considered sleep as developing from the primitive rest states in animals, and such animals were able to survive the struggle for existence as were able to acquire it. Sleep is an instinct, designed to protect the organism from the effects of fatigue; we sleep, not because we are exhausted or fatigued but because sleep is necessary for existence.

Of the more recent theories, psycho-biological in character, that of Sidis1 offers most promise. By exposing various subjects to the effects of a monotonous stimulus, as the beats of a metronome, by limiting voluntary movements, and by shutting out sensory stimuli, as by closing the eyes, he arrived at the conclusion that sleep is dependent upon three factors chiefly, - monotony of sensory impressions, limitation of voluntary movements, inhibition of sensory stimuli; the first two were considered of most importance. According to this view, we sleep when stimuli have, by reason of their monotony, exhausted their ability to elicit a response from consciousness. Like Claparede, Sidis favours the evolutionary development of sleep.

1 From Journal of Abnormal Psychology, 1908, Richard Q. Badger, Publisher; Balliere, Tindall & Cox, London.

That this theory is not free from objection is shown by the arguments of Coriat.1 According to this experimenter, neither monotony of sensory impressions nor limitation of voluntary movements are necessary for sleep. In his opinion, based on numerous experiments on animals and humans, muscular relaxation and disinterest are the main requirements. The value of muscular relaxation is emphasized; this shuts out a mass of stimuli which would otherwise reach the brain from the muscular system, and which aid in keeping up the activity of consciousness. Whenever a state of muscular tension, however slight, was induced in his subjects, sleep failed to be produced.

Doubtless, with the muscles in a state of tension, conscious attention is kept awake and hence sleep may be defeated. However, that muscular relaxation is not absolutely necessary is shown by the fact that soldiers not infrequently sleep standing, and while on a march, and most of us must know of instances of persons sleeping while walking, riding horseback, etc. Further, it is natural for many animals to sleep in a position necessitating muscle tonus. Elephants sleep standing up, and when in a herd, one or two keep awake on sentry duty; bats sleep head downwards, hanging by their hind claws; storks and other long-legged birds sleep standing on one leg; ducks keep paddling to prevent drifting to shore; the potto, a distant relative of the monkey, sleeps with its head tucked under its body and clinging to a limb; horses and many other animals often sleep standing, and instances have been reported of horses which, under constant watchfulness day and night, were not known to have been recumbent for as long as thirteen years.