Partial rest is the same thing as change of occupation, and by a careful regulation of the relative amount of cerebral and muscular work, energy can be largely economised. The horse, which exercises chiefly his muscles, requires only five or six hours to recuperate his energy; and our muscles require less sleep than our brain.
Sleep is the only form of complete and general rest. In attaining this condition, the muscles sleep first, then the eyes close (owing to muscular rest), and the thoughts wander; hearing is the last sense to lose cognizance of the surrounding world; dreaming succeeds wandering thoughts, and even dreaming may cease if the brain repose is complete.
During sleep the brain diminishes in size, and becomes paler; the amount of blood in the brain being diminished. Probably the cerebral anæmia is rather a consequence of the functional inactivity of the brain during sleep than a cause of the sleep.
During sleep the heart and lungs continue their work; the blood is circulated and purified, the intestines continue their vermicular contractions, and absorb food from the alimentary canal, and the organs nourish themselves at leisure.
Two facts relating to sleep have important practical bearings. First, during sleep metabolism is less active, and so the temperature of the body tends to be somewhat lowered. Secondly, assimilation is more energetic; this favours the absorption of noxious vapours, if any are present. There is probably, therefore, slightly less danger of remaining in a stuffy, impure atmosphere during the day than at night.
Practical Rules Concerning Sleep.—1. Amount of sleep required. It is impossible to lay down any fixed rule applicable to all persons and circumstances. The amount of sleep required, like the amount of food, varies greatly.
Habitual deficiency of sleep produces a condition of wretchedness and prostration, with great restlessness. Prolonged watching inevitably breaks down the constitution. Not the least evil consequence of irregular and deficient sleep is, that sleep, when desired, is often courted in vain.
Habitual excess of sleep produces a condition of brain less active than usual, and less favourable for thought and action. Impressions are received less readily, and the power of will is correspondingly diminished.
The amount of sleep required varies with—
(1) Age.—The infant, if healthy, spends the larger part of his existence in sleep; gradually the amount required diminishes until, for the adult, seven or eight hours suffice. Children over two or three years old require sleep only during the night. In advanced life there is a tendency to revert to infantile habits, sleep occurring in frequent short snatches.