The Encyclopedia Americana (1920)/Sleep
SLEEP, a state of rest for both body and mind, during which there is partial abeyance of volition and consciousness, with a relaxed condition of the body and a lessened activity of certain of the vital functions. The muscles do less work in the way of voluntary contractions, and there is, therefore, a diminution in this feature of their tonicity; the respirations are less frequent and rhythmical; occasionally there is a long expiratory pause; the output of carbon dioxide is diminished more than the consumption of oxygen; the pulse is less frequent; the secretions are partially suspended, and the pupils of the eyes are contracted. The whole body is in a state of increased vagus tonus, vagotonia.
Sleep is probably due to a rhythmical cycle habit of millions of years of evolution and is chiefly a tropism reaction to light and temperature impressions. It has been biologically developed by reason of its value as a resting period for less active energy transformation. All parts of the body require such rest as sleep affords, and share, directly or indirectly, in its benefits, for functional activity without periods of rest results in an undue destruction of the tissues and an excess of poisonous products. The essence of the sleep state is functional rest and nutritive repair. Though the brain is in repose, it nevertheless is in function during sleep just as much as all the rest of the body. This activity is registered in dream activity, some of which remain in consciousness. Generally, the more the body can be withdrawn from outside influences the more readily is refreshing sleep obtained. Sometimes pain assists to this end. For example, a man is worried with business cares and has not slept well of late. One evening he has a severe toothache which demands his entire attention. As the pain is relieved he drops off into a quiet sleep and forgets his troubles in unconsciousness.
The amount of sleep needed by different persons varies with age and condition. The greater part of infancy is generally passed in slumber, which is more profound than that of adults. In middle life from six to eight hours' sleep a day is necessary, though it is reported of Frederick the Great and Napoleon that they slept but three or four hours daily. Old persons frequently require seven or more hours' sleep, though some live healthfully with but six. Occupation, race, sex, climate and habit have to do with the amount of sleep required. Brain workers, as a class, take less sleep than laborers. Sleep is sounder and longer in cold climates and among northern races. Women usually take more sleep than men. The influence of habit is noticeable when the demand for sleep comes at a definite time, or in the case of those accustomed to sleep amid noisy surroundings, and who cannot readily sleep in a quiet resting-place.
The disorders of sleep may be classified as follows: (1) Absence of sleep — insomnia, vigilance; (2) imperfect or partial sleep — dreams, sleep-drunkenness, or somnolentia, night-terrors and nightmare; (3) perverted or artificial sleep — somnambulism, hypnotism, mesmerism, trance, somnium; (4) excessive or frequent drowsiness and sleep — morbid drowsiness or somnolence, paroxysmal sleep, epileptic sleeping attacks, trance-sleep, cams, cataphora, lethargy — sleeping-sickness of Africa.
The causes of disordered sleep are many, such as brain affections, blood-poisoning, pain, indigestible food, discomfort, produced by undue cold or heat, mental excitement or worry, overfatigue, febrile conditions, sleeping in an uneasy position or with the head too high or too low, too little outdoor exercise, sleeping amid unaccustomed surroundings. The treatment of disturbed sleep sometimes taxes the art of the physician. Mental repose, bodily comfort, a sufficient degree of warmth, a certain amount of fatigue, combined with perfect quietude, are essential. An evening walk, a cup of hot milk or cocoa, or a weak stimulant just before retiring, massage, counting and verbal repetitions are aids in producing sleep. Hypnotic medicines should be avoided if possible. Brisk exercise in the open air, every day if convenient, to the point of tiring, is a good sleep-producer. Consult Powell, L. P., ‘Art of Natural Sleep’ (New York 1908); Bruce, H. A., ‘Sleep and Sleeplessness’ (Boston 1915).