286.  The Sabbath as a Day of Physiological Rest.  The divine institution of a Sabbath of rest, one day in seven, is based upon the highest needs of our nature.  Rest, to be most effective, should alternate in brief periods with labor.

It is sound physiology, as well as good morals and manners, to cease from the usual routine of six days of mental or physical work, and rest both the mind and the body on the seventh.  Those who have succeeded best in what they have undertaken, and who have enjoyed sound health during a long and useful life, have studiously lived up to the mandates of this great physiological law.  It is by no means certain that the tendency nowadays to devote the Sabbath to long trips on the bicycle, tiresome excursions by land and sea, and sight-seeing generally, affords that real rest from a physiological point of view which nature demands after six days of well-directed manual or mental labor.

287.  The Significance of Sleep as a Periodical Rest.  Of the chief characteristics of all living beings none is so significant as their periodicity.  Plants as well as animals exhibit this periodic character.  Thus plants have their annual as well as daily periods of activity and inactivity.  Hibernating animals pass the winter in a condition of unconsciousness only to have their functions of activity restored in early spring.  Human beings also present many instances of a periodic character, many of which have been mentioned in the preceding pages.  Thus we have learned that the heart has its regular alternating periods of work and rest.  After every expiration from the lungs there is a pause before the next inspiration begins.

Now sleep is just another manifestation of this periodic and physiological rest by which Nature refreshes us.  It is during the periods of sleep that the energy expended in the activities of the waking hours is mainly renewed.  In our waking moments the mind is kept incessantly active by the demands made on it through the senses.  There is a never-ceasing expenditure of energy and a consequent waste which must be repaired.  A time soon comes when the brain cells fail to respond to the demand, and sleep must supervene.  However resolutely we may resist this demand, Nature, in her relentless way, puts us to sleep, no matter what objects are brought before the mind with a view to retain its attention.[41]

288.  Effect of Sleep upon the Bodily Functions.  In all the higher animals, the central nervous system enters once at least in the twenty-four hours into the condition of rest which we call sleep.  Inasmuch as the most important modifications of this function are observed in connection with the cerebro-spinal system, a brief consideration of the subject is properly studied in this chapter.  In Chapter IV. we learned that repose was as necessary as exercise to maintain muscular vigor.  So after prolonged mental exertion, or in fact any effort which involves an expenditure