The principle of excitability exists in the highest degree in the infant, and diminishes at every succeeding period of life; and if man is not cut down by disease or violence, he struggles on, and finally dies a natural death--a death occasioned by the exhaustion of the principle of excitability. In order to prevent the too rapid exhaustion of this principle, nature has especially provided for its restoration by establishing a period of sleep. After being awake for sixteen or eighteen hours, a sensation of fatigue ensues, and all the functions are performed with diminished precision and energy. Locomotion becomes feeble and tottering, the voice harsh, the intellect obtuse and powerless, and all the senses blunted. In this state the individual anxiously retires from the light, and from the noise and bustle of business, seeks that position which requires the least effort to sustain it, and abandons himself to rest. The will ceases to act, and he loses in succession all the senses; the muscles unbend themselves, and permit the limbs to fall into the most easy and natural position; digestion, respiration, circulation, secretion, and the other functions, go on with diminished power and activity; and consequently the wasted excitability is gradually restored. After a repose of six of eight hours, this principle becomes accumulated to its full measure, and the individual awakes and finds his system invigorated and refreshed. His muscular power is augmented, his senses are acute and discriminating, his intellect active and eager for labor, and all his functions move on with renewed energy. But if the stomach be oppressed by food, or the system excited by stimulating drinks, the sleep, though it may be profound, is never tranquil and refreshing. The system being raised to a state of feverish excitement, and its healthy balance disturbed, its exhausted excitability is not restored. The individual awakes, but finds himself fatigued rather than invigorated. His muscles are relaxed, his senses obtuse, his intellect impaired, and his whole system disordered; and it is not till he is again under the influence of food and stimulus that he is fit for the occupations of life. And thus he loses the benefits of this wise provision of repose, designed for his own preservation. Nothing, probably, tends more powerfully to produce premature old age, than disturbed and unrefreshing sleep. It is also true, that artificial stimulus, in whatever way applied, tends constantly to exhaust the principle of excitability of the system, and this in proportion to its intensity, and the freedom with which it is applied.