You are aware that many remedies in medicine may be so intense as to cause fatality: inflammation, too, is the restorative process of wounds, but if in excess it is fatal. Appetite also, to a certain degree, is healthy; but craving and thirst, its extremes, are proved, by suffering, to be morbid.
If the mind is composed to perfect rest, it is lulled to senselessness; then metaphysically we are said to sleep: the mind is not excited by thought, and, in consequence, its supply of arterial blood is less, the more rapid flow of which would be the cause of waking.
Within certain limits sleep is a remedy; but it becomes perilous when intense, or too much indulged. One eccentric physician, as we read in the learned Boërhave, even fancied sleep the natural condition of man, and was wont to yield to its influence during eighteen of the twenty-four hours; but apoplexy soon finished his experiment.
This negative quiescence (for sleep is not a positive state) allows the restoration of energy, and then we wake. Even the senses accumulate their power in sleep; the eye is dazzled by the light when we wake, from the sensitiveness imparted by this accumulation.
The conceits regarding the cause of sleep are so various, that if I were to discuss their merits I should only weary your patience, as I perceive I have already done.
Some have thought that sleep arose from certain conditions of the blood in the vessels and nerves of the brain; its congestion in the sinuses; or a reflux of a great portion of it towards the heart: the result of depressed nervous energy—exhaustion, fatigue, cold, and the influence of powerful narcotics, or the combustion of charcoal. Others, that sleep arises from the deposition of fresh matter on the brain, and its sudden pressure. Then we have the cerebral collapse of Cullen, and of Richerand; the deficiency of animal spirits of Haller; the diminished afflux of blood to the brain of Blumenbach; and the exhausted irritability of the Brunonian theory adopted by Darwin.
Where the truth lies I presume not to decide, but it is clear there is a necessity for the occasional repose of the mental organ:
“Non semper arcum
Tendit Apollo.”
Watchfulness invariably reduces, even in the brute: the wild elephant is tamed by the perseverance of the hunter in keeping it constantly awake.