ursions 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 of what is often called nerve-force, sleep becomes a
necessity. The need of such a rest is self-evident, and the loss of it is
a common cause of the impairment of health. While we are awake and active,
the waste of the body exceeds the repair; but when aslee