too, for long periods. Some sleep lightly, retaining in greater part their consciousness. Occasionally we hear of an individual who has lived for a long time without sleep, so far as can be determined, and yet has continued to maintain good health. Sufferers from one form or another of nervous exhaustion are often compelled to forego sleep temporarily. Vigorous persons of pronounced personality and highly developed consciousness have the least need for sleep, at least while at the zenith of their powers and in the full flower of energizing.
The maintenance of conscious life demands an expenditure of energy so intense that the processes of nutrition and reconstruction of cellular waste can not be carried on without sleep. Complete repose of the consciousness is demanded for the plastic nutrition of the organism and the accomplishment of vegetative life. Consciousness is the highest of our faculties, rendering possible moral and scientific ideation; it demands the greatest efforts of our organism. In its absence sleep is less required.
All the internal organs are, during sleep, relatively less filled with blood because then the skin is in a state of hyperemia or gorged with blood. The sweat glands act more energetically at night whether we are asleep or awake, hence the danger of chills is then greater. All the organic activities continue, but are less vigorous at night, and during sleep, whereas during sleep in daylight hours these proceed with little alteration. When animals or men feel the desire to sleep they instinctively seek a quiet sheltered spot, as free as possible from light and noise, thus avoiding whatever impressions from the external world are liable to be subjectively translated into sensations. The eyelids are lowered; a position is sought wherein the muscles can be fully relaxed. The sensorial organs are capable of acting during sleep and continue to transmit impressions into conscious sensations.
With the pallor of the brain, which occurs in sleeping animals, the cortex ceases to react so readily to mechanical, photic, electric or other stimuli. The spinal cord and sensory nerves do not sleep, yet sensations of pain are then lowered. The nerves transmit painful impressions, but the consciousness of the sleeper perceives them incompletely. The voluntary muscles become quiescent during sleep, but retain their power, as shown by the normal subject in changing position, arranging the bedclothes, even walking; soldiers are able to march or ride while asleep. The brain is the chief part which sleeps, but it is not wholly inactive, exciting inhibitions which check the formation of reflex movements. If stimuli are applied of sufficient intensity to overcome the protective states of the somnolent consciousness the subject awakes, recognizing the cause more or less certainly.
Sleep is not an absolute arrest of cerebral activity; the brain then retains always partial energy. In deprivation of sleep it is the brain
