SLEEP. Sleep is a normal condition of the body which occurs periodically. During sleep there is greater or less depression of most physiological activities, accompanied by a greater or less degree of unconsciousness. Until the present century, neither physiologists nor psychologists have been able to offer any expla nation of the cause or nature of normal sleep.
Sleep as we know it in man is observed only in those animals which stand on a comparatively high level of development as regards their central nervous system. It is however possible that the periods of rest in the daily cycle of some invertebrates are of a nature akin to that of sleep. Amongst the vertebrates, even fishes show unmistakably a periodical sleep, and in birds this assumes all the characteristics of the typical sleep of a mammal.
Bodily Manifestations of Sleep.—The most striking mani festation of sleep is the partial or complete depression of the higher nervous activities, a state which is subjectively described as unconsciousness. However it is not only the higher parts of the central nervous system that are involved. Those parts which regulate posture and the tonic state of the skeletal muscles, and even those which regulate the respiratory movements and tone of the blood vessels are also found to be depressed during sleep.
As the result of an almost complete relaxation of the muscles, all signs of muscular activity diminish. The general metabolism is depressed and reaches the lowest level of the day. If we take the average daily metabolism of an adult as about 3,00o calories, only about 600 of these will be used during 8 hours of sleep, about 75o during 8 hours of the wakeful state at rest, and about 1,65o during 8 hours of mild muscular activity. As regards the state of metabolism we may, therefore, regard sleep as equivalent to almost complete rest of the skeletal muscles. Since the process of reconstruction of organs, i.e., their recovery after a preceding period of activity, can take place only in the absence of these activities, it is natural that sleep should afford a better opportu nity for recovery from fatigue than simple rest, in which the muscles are never in as complete relaxation as during sleep.
Whether the metabolism of other organs is also diminished during sleep has not yet been ascertained. It is known, however, that all digestive processes, such as the secretion of the digestive juices, the movements of the gastrointestinal tract and the absorption of food substances, proceed in a normal manner ; but these functions are not so intimately dependent on the central nervous system as are those of the skeletal muscles.
Under normal conditions in the state of wakefulness, the regu latory functions of the nervous system are so delicately adjusted that they tend, by various mechanisms, to counteract any violent changes in the internal conditions of the body which would other wise be caused by changes in the environment. A large number of the mechanisms involved in this "self preservation" of the organism are dependent on the proper functioning of the central nervous system, and in sleep, on account of the depression of this system, the compensatory activities of the organism are not at such a high pitch of efficiency as in wakefulness.
Temperature.—The temperature regulation is less efficient : the diminished production of heat is not completely compensated by a diminution of heat loss, and the temperature of the organism falls. Thus, exposure to cold may lead to a considerably greater change of body temperature in sleep than otherwise. This rela tively greater cooling of the organism may probably start some time before actual sleep has commenced. This may be the cause of the subjective feelings of cold if sleep is delayed. It is well known that the temperature of man exhibits daily fluctuations; it is usually highest about 5 to 7 P.M. and falls during the night, reaching the lowest point early in the morning. These variations in temperature may amount to a degree or more Centigrade.