ight 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 would be
quite run down: how properly therefore has our great poet called
sleep "the 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 in some way or other rendered unfit to
transmit to them the impulses of bodies during the suspension of the
power of volition; thus the eyelids are closed, in sleep, to prevent
the impulse of light from acting on the optic nerve; and it is very
probable that the drum of the ear is not stretched; it seems likewise
reasonable to conclude, 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 these powers have acted, the less are we
inclined to sleep. Instances of the first are, excess of exercise,
strong liquors, or study; and of the latter, an under or deficient
proportion of these.

A person who has been daily accustomed to much exercise, whether
mental or corporeal, if he omit it, will find little or no
inclination to sleep; this state may however be induced by taking
some diffusible stimulus, as a little spirits and water, or opium,
which seem to act entirely by exhausting the excitability, to that
degree which is compatible with sleep, and, when the stimulant effect
of these substances are over, the person soon falls into that state.

But though the excitability may have been sufficiently exhausted, and
the action of external powers considerably moderated, yet there are
some things within ourselves, which often stimulate violently, and
prevent sleep, such as pain, thirst, and strong passions and emotions
of the mind. These all tend to drive away sleep, by the