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 asleep, the waste is diminished, and the cells are more actively rebuilding the structure for to-morrow’s labor.  The organic functions, such as are under the direct control of the sympathetic nervous system,—­circulation, respiration, and digestion,—­are diminished in activity during sleep.  The pulsations of the heart and the respiratory movements are less frequent, and the circulation is slower.  The bodily temperature is reduced, and the cerebral circulation is diminished.  The eyes are turned upward and inward, and the pupils are contracted.

The senses do not all fall to sleep at once, but drop off successively:  first the sight, then the smell, the taste, the hearing and lastly the touch.  The sleep ended, they awake in an inverse order, touch, hearing, taste, smell, and sight.

289.  The Amount of Sleep Required.  No precise rule can be laid down concerning the amount of sleep required.  It varies with age, occupation, temperament, and climate to a certain extent.  An infant whose main business it is to grow spends the greater part of its time in sound sleep.  Adults of average age who work hard with their hands or brain, under perfectly normal physiological conditions, usually require at least eight hours of sleep.  Some need less, but few require more.  Personal peculiarities, and perhaps habit to a great extent, exert a marked influence.  Some of the greatest men, as Napoleon I., have been very sparing sleepers.  Throughout his long and active life, Frederick the Great never slept more than five or six hours in the twenty-four.  On the other hand, some of the busiest brain-workers who lived to old age, as William Cullen Bryant and Henry Ward Beecher, required and took care to secure at least eight or nine hours of sound sleep every night.

In old age, less sleep is usually required than in adult life, while the aged may pass much of their time in sleep.  In fact, each person learns by experience how much sleep is necessary.  There is no one thing which more unfits one for prolonged mental or physical effort than the loss of natural rest.

290.  Practical Rules about Sleep.  Children should not be played with boisterously just before the bedtime hour, nor their minds excited with weird goblin stories, or a long time may pass before the wide-open eyes and agitated nerves become composed to slumber.  Disturbed or insufficient sleep is a potent factor towards producing a fretful, irritable child.

At all ages the last hour before sleep should, if possible, be spent quietly, to smooth the way towards sound and refreshing rest.  The sleep induced by medicine is very often troubled and unsatisfactory.  Medicines of this sort should not be taken except on the advice of a physician.