Sleep

They have been shown to be independent of the cosmic phenom ena of day and night, and directly related to sleep and wake fulness. The most conclusive proof of this is that the temperature of people who work during the night and sleep during the day also falls during the periods of sleep, the highest temperature occurring at night which is, in these cases the period of greatest activity.

The regulation of temperature is to a large extent due to muscular activity, and since this is depressed during sleep, the regulation of temperature is not at all adequate during periods of sleep. This compels us to seek warmer covering during sleep, while animals try to diminish their exposed surfaces by curling up, etc. Con versely, the animal is to a large extent hindered from falling asleep by any conditions which provoke an increase in metab olism, exposure to cold being one of them.

Respiration.—The respiration during sleep becomes slower and deeper. It is reported that it becomes predominantly costal, the diaphragmatic contractions becoming considerably weaker than they are normally. Often the respiratory movements become periodical in character, tending to rise and fall in strength. The depression of respiration is such that, although the production of carbon dioxide in sleep is considerably smaller, the concentration of carbon dioxide in the alveolar air is unmistakably increased.

A similar increase during the wakeful state would cause an imme diate augmentation of the respiratory activity. The nitrogen elimination remains unchanged, which means that the protein metabolism is not affected.

Heart Action.—The heart rate during sleep is slower, but it is not known whether this is due to an increased tone of the cardio inhibitory nerves, or to a diminished tone of the accelerator nerves. The vasomotor centre is also depressed, as the result of which the peripheral blood vessels dilate and the arterial blood pressure declines. The cutaneous blood vessels largely participate in this vasodilation so that in some cases pulsation of the veins can be observed during sleep. The blood pressure in very deep sleep exhibits little fluctuation, except for the usual respiratory variations, but in sleep which is not profound changes in the blood pressure have been observed which may exceed those which occur during ordinary resting conditions. The nature of these fluctua tions is not clear. They may occur spontaneously and may pos sibly be dependent on the nervous activities associated with dreams. Interesting experiments were carried out on the vol ume occupied by a man's arm during sleep. It was found that the arm increased in volume on account of vasodilation. After the

maximum dilation is reached, the arm remains more or less of the same volume for a certain period, or else the volume diminishes very gradually. Shortly bef ore the awakening, the arm begins to diminish more rapidly in size, doubtless owing to the contraction of its blood vessels. At the time of waking, it has practically the same volume as it had at the beginning of sleep. On account of dilation of the peripheral blood vessels during sleep, there is a considerable redistribution of blood ; less blood is now available for the brain, and possibly less for some other internal organs.

Brain Circulation.--The

state of the circulation of the brain has frequently been investigated. It has been observed that during sleep the surface of the exposed brain becomes pale (Johann Blumenbach 1752-1840). A careful research was conducted by A. E. Durham in 186o, in which he trephined a portion of bone in the parietal region of a dog and inserted a watch-glass in the open ing to prevent the effects of atmospheric pressure. He found that during sleep the brain is relatively more anaemic. Other experi ments confirmed these observations, and Mosso (1881) extended the experiments to man. He made observations on three persons who had lost a portion of the cranial vault, and in whom there was a soft pulsating cicatrix. The subjects were a woman of thirty seven years of age, a man of thirty seven years of age, and a child of about twelve years. By special arrangements, Mosso took simultaneous tracings of the pulse at the wrist, of the beat of the heart, of the movements of the wall of the chest in respiration, and of the movements of the denuded brain. Further, he obtained tracings showing changes in the volume of the hand and forearm; and he succeeded in showing that during sleep there is a dimin ished amount of blood in the brain, and at the same time an in creased amount in the extremities. The blood flow through the retina is also diminished during sleep. The apparent explanation of the cerebral anaemia during sleep is that the blood vessels of the body dilate, and therefore receive more blood, while a smaller amount flows to the brain; it is not due to an active constriction of the cerebral blood vessels. Whilst muscular relaxation is gen eral, there seems to be an increased contraction of certain sphinc ter muscles, such as the circular fibres of the iris and the fibres concerned in closing the eyelids; the pupils of the eyes are contracted and the eyeballs are rotated upwards.