CHAPTER XXXVIII.

PERSONAL HYGIENE (_continued_)—REST AND SLEEP.

=Physiological Considerations.=—Life is made up of alternations of rest and action. The exercise of any organ is followed by a necessary period of repose, during which the oxidised materials produced by functional activity are removed by the blood, and carried to the excretory organs; while at the same time fresh nutritive material is supplied by the blood to make good the losses thus sustained.

The only apparent exceptions to this rule of alternation of rest and exercise are the heart and lungs, and some less important organs acting out of the control of personal volition. But even these organs obey the universal law. The difference is that their rest is very frequent and momentary; the heart having to contract sixty or seventy times per minute, rests 6∕11 of a second each second, or more than thirteen hours in the twenty-four. The lungs and respiratory muscles rest a shorter time than this, but probably about three hours per day.

The necessity for rest is well shown by the sense of taste. If salt is kept in the mouth for a considerable time, the power of tasting it disappears, and only returns in its original strength after several hours. The gustatory nerve has been exhausted.

The other sense-organs illustrate the same principle. Persons are not uncommonly made deaf by the sounds of machinery. After looking at a particular colour for some time, the nerves receiving impressions from this colour are exhausted, and only its complementary colour is visible.

Rest may be either _partial_ or _general_.

The principle of partial rest has very useful practical bearings. Such rest is illustrated by the student who takes a walk, or uses methodical gymnastic exercises; a concert may provide agreeable exercise for the auditory nerves and the part of the brain connected with them, while allowing the over-tired intellectual part of the brain to rest in peace; similarly, light literature may prove a pleasing rest after severer studies.

Walking is more especially the exercise of the brain-worker.

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.

(2) _Sex._—Women have been stated to require rather more sleep than men, but this is doubtful. The hours of sleep required have in accordance with this view been stated to be, “Six for a man, seven for a woman, and eight for a fool.” A reversal of this order would more nearly approximate to the requirements of town life.

(3) _Temperament._—Those of a cold lymphatic temperament require more sleep than sanguine or nervous people, though the latter sleep more deeply. Frederick the Great, John Hunter, and Napoleon I. are said to have required only five hours’ sleep per day; but the last of these had the faculty of taking short naps at a few moments’ notice.

(4) _The sick and convalescent_ require much more sleep than those who are healthy.

(5) _Habit_ has a very important influence. Many people appear to sleep too much, and thus dull to some extent their mental faculties; but on the other hand, modern life, with its nervous strain, keen competition, and constant hurry and worry may make a larger amount of sleep necessary than that required by our forefathers, who invented the foregoing proverb.

(6) _Occupation._—Mental work requires more repose than physical.

2. _Relation of sleep to food._—The molecular life of the tissues—that is, the processes of nutrition—ought to be undisturbed. These go on most perfectly when no active function, such as that of digestion, is being performed. But while the stomach carries on the digestive functions to only a small extent during sleep, the intestines continue still to digest and absorb food. In accordance with these facts, it is advisable to allow at least two hours between the last meal of the day and sleep, especially if animal food has been taken.

3. As absorption is increased and the temperature is lowered during sleep, it is important to _sleep in pure air_, and to have _warm coverings_, especially about the shoulders and arms. Many an obstinate cough might be cured by the simple expedient of wearing a flannel jacket at night.

4. _Sleep during the night and not during the day._ It should hardly be necessary to say this, as the universal instinct of animals shows its advisability; but, unfortunately, the habits of mankind have commonly led to a partial reversal of the natural arrangement.

5. The room should be dark; light, like sound, is inimical to sleep. The head should be moderately raised. The temperature of the room for robust persons need not be artificially raised.

=Sleeplessness=, as a rule, occurs only when some physiological law has been broken. To relieve it, it is essential to equilibrate muscular and mental functions. Increase of muscular exercise is an important element in its treatment. In addition it is advisable not to have any severe mental work during the evening, nor to indulge in late suppers. Sleeplessness is the bane of many men of a nervous temperament, and chiefly attacks those of sedentary habits. It is apt to recur, and for this reason, if for no other, narcotics ought to be scrupulously avoided. The habit of taking such soporifics is unfortunately becoming much more common, and is productive of many evils. Death from accidental overdose is a frequent calamity; and, apart from this possibility, the invalid’s nervous system is completely ruined by persistence in the habit, his power of will is annihilated, and he becomes the miserable slave of an evil habit, whose end is death (see also page 54).