These are examples of the finite, or irreparable exhaustion of the excitability, but we find also, that it may be exhausted for a time, and accumulated again.  Though the eye has been so dazzled by the splendour of light, that it cannot see an object moderately illuminated, yet, if it be shut for some time, the excitability of the optic nerve accumulates again, and we are again capable of seeing with an ordinary light.

We find, that we are not always equally capable of performing the functions of life.  When we have been engaged in any exertion, either mental or corporeal, for some hours only, we find ourselves fatigued, and unfit to pursue our labours much longer; if in this state, several of the exciting powers, particularly light and noise, be withdrawn; and if we are laid in a posture which does not require much muscular exertion, we soon fall into that state which nature intended for the accumulation of the excitability, and which we call Sleep.  In this state, many of the exciting powers cannot act upon us, unless applied with some violence, for we are insensible to their moderate action.  A moderate light, or a moderate noise, does not affect us, and the power of thinking, which exhausts the excitability very much, is in a great measure suspended.  When the action of these powers has been suspended for six or eight hours, the excitability is again capable of being acted on, and we rise fresh, and vigorous, and fit to engage in our occupations.

Sleep then, is the method which nature has provided to repair the exhausted constitution, and restore the vital energy; without its refreshing aid, our worn-out habits would scarcely be able to drag on a few days, or at most a few weeks, before the vital spring was quite run down; how properly therefore has the great poet of nature called sleep the chief nourisher in life’s feast.—­

’Sleep, that knits up the ravell’d sleeve of care, ’the death of each day’s life, sore labour’s bath, ’balm of hurt minds, great Nature’s second course, ‘chief nourisher in life’s feast.’

From the internal sensations often excited, it is natural to conclude that the nerves of sense are not torpid during sleep; but that they are only precluded from the perception of external objects, by the external organs being rendered unfit to transmit to them the impulses of bodies, during the suspension of the power of volition; thus, the eye-lids are closed in sleep, to prevent the impulse of the light from acting on the optic nerve; and it is very probable that the drum of the ear is not stretched; it is likewise probable that something similar happens to the external apparatus of all our organs of sense, which may make them unfit for their office of perception during sleep.

The more violently the exciting powers have acted, the sooner is sleep brought on; because the excitability is sooner exhausted, and therefore, sooner requires the means of renewing it; and on the contrary, the more weakly the exciting powers have acted, the less is a person inclined to sleep.  Instances of the first are, excess of exercise, strong liquors, or study, and of the latter, an under proportion of these.