Thus We may in short look upon the time of _Watching_, as the time of Wearing out, or the Destruction of the Animal Fabrick; and the time of _Sleep_, as that in which it is repaired and recruited; not only upon the account of what We have just mentioned concerning the _Nervous Liquor_, but also with respect to all the other Parts, as well Fluid as Solid. For _Action_ does necessarily by Degrees impair the Springs and Organs; and in _Motion_ something is continually abraded and struck off from the Distractile Fibres, which cannot otherwise be restored than by their being at rest from _Tension_. Besides that, such a regular and steady _Course_ of the Blood, as we have observed to be in _Sleep_, is by far more fit and proper for Nutrition, or an Apposition of Parts to the Vessels, which an uneven _Hurry_ of It is more apt to tear off and wash away. The Case being thus, it is very plain that whatsoever can induce such a Disposition of the Fluids and Muscular Parts of the Body, as this We have described, will so far cause _Sleepiness_. And in like manner, when any thing interposes and hinders this Composedness and Tranquillity, the removing of the _Impediment_ will be the causing of _Sleep_; inasmuch as this is only reducing the Animal _Œconomy_ to its right State, in which by natural Order there must be a Succession of Sleeping and Waking. Thus it appears how necessarily continued Exercises do make Us _Sleepy_, since These do exhaust the Juice of the Nerves; that is, both lessen its Influx into the Organs of Motion, and incline the Mind not to determine it any longer that way, upon the account of the Pain and Uneasiness, with which too violent a Tension of the Parst is always attended; which therefore we must needs desire to Relax, or lay to _Rest_. That _Sleepiness_ which follows upon a fulness of the Stomach after Eating or Drinking, is owing to a different Cause; and does indeed so nearly fall in with the Effects of _Opiate_ Medicines, that it requires a particular Consideration. As _Hunger_, or the Emptiness of the Stomach, is a painful Sensation; so the satisfying or removing of _This_, is a pleasing or agreeable One. Now all _Pain_ is a _Stimulus_ upon the Part affected; and This, we all know, being attended with Contractions of the pained Membranes, causes a greater Afflux than ordinary of the Nervous Juice that way. On the other Hand, _Pleasure_, or a delightful Sensation in any part, is accompanied with a smooth Undulation, and easie Reflux of the Liquor of the Nerves towards the Brain. This is, as it were, the _Entertainment_ of the Mind, with which being _Taken up_, it does not Determine the Spirits to the Organs of Motion; That is, there is such a _Relaxation_ of the Muscular Fibres, and such a Disposition of the Nervous Fluid, as we have observed to be necessary to Sleep. This is the Reason of that _Chilliness_ in the Limbs, which we commonly Complain of after a good Feast.