Sleep, a period of repose in the animal system, in which there is a partial suspension of nervous and muscular activity, necessary for the reparation of the vital powers. In sleep there is more or less complete unconsciousness of external impressions, which may be dissipated by any extraordinary excitement, in this respect differing from the torpor of coma produced by abnormal conditions within the cranium or the action of narcotic poisons. In the deep sleep after extreme fatigue there may possibly be a complete suspension of the activity of the cerebrum and the sensory ganglia; some consider dreams a proof of imperfect sleep, while others maintain that there are always dreams during sleep, though they may not be remembered. The refreshing power of sleep depends on the nutritive renovation effected during its continuance; it is a necessity of the system, and must be periodically indulged in. After 12 to 1G hours of waking a sense of fatigue is experienced under ordinary circumstances, showing that the brain needs rest, and this cannot be shaken off unless by some strong physical or moral stimulus; more sleep is required by the young, and less by the aged, in proportion to the rapidity of waste of the tissues.

When the sense of fatigue has reached its maximum, sleep will supervene, even under the most unfavorable circumstances. It may be retarded by uncommon mental concentration, excitement, suspense, or the exercise of a strong will, but always with an exhaustion of nervons power which requires a proportionally long period of repose. Stillness, the absence of light, and monotonous low noises, like the buzzing of insects, the murmur of the wind in the trees, the purling sound of running water, the rippling on a beach, the suppressed hum of a distant town, the droning voice of a dull reader, or the mother's lullaby, promote sleep; gentle movements, like the swinging of a hammock or the rocking of a cradle or boat, are also conducive to sleep; in reading a dull book the eyes wander fatigued from page to page, and the excitement of the mind is not enough to overcome the tendency to sleep. Persons may become so accustomed to continuous loud noises, as in the vicinity of mills, forges, and factories, that they cannot readily fall asleep in their absence. The transition from sleep to the waking state, and vice versa, is generally gradual, but sometimes sudden.

The foetus may be said to be in a continued sleep, and the excess of the sleeping over the waking hours prevails during infancy and childhood, or while growth is greater than the decay of the tissues, and this sleep is more profound as well as longer. Persons of plethoric habit, with good appetite and powers of digestion, are usually sound sleepers; the nervous sleep comparatively little ,• lymphatic, passionless individuals, who vegetate rather than live, are generally long sleepers. The amount of sleep required depends much on constitution and habit, and the smallest sleepers have sometimes been men of the greatest mental activity. Most men require from six to eight hours of sleep daily, and this amount cannot be materially diminished without injury to the health. As a general rule, the amount necessary to refresh the system is in proportion to the amount of bodily and mental exertion of the individual. - In natural sleep, during the repose of the voluntary muscles, the senses, and the perceptive and intellectual faculties, the functions of respiration, circulation, nutrition, secretion, and'absorption continue. The respiration and the pulse, however, are both diminished in frequency; and the temperature of the body is somewhat reduced from its usual standard.

Hence the chilliness generally felt during a nap in the daytime, and the propriety of throwing some covering over the body during sleep, even in summer, to avoid taking cold; in this state there is also less power of resisting diseases, especially malarious ones. Nothing is so refreshing during sickness, or so conducive to rapid convalescence, as quiet sleep; and few symptoms are more unfavorable than continued sleeplessness. A habitual deficiency of sleep, from excitement or excessive study, produces sooner or later headache, cerebral disturbance, restlessness and feverishness, and, if the warning be not seasonably heeded, a serious impairment of the vital powers. (See Coma, Dream, and Somnambulism).