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