Recall that in the 1730s you held to a theory of pores, with both excreting skin pores to let out "the perspirable
matter" and imbibing pores that drew in liquids.
Confined air, when saturated and perspirable matter, will not receive more; and that matter must remain in our bodies, and occasion diseases, but it gives some previous notice of its being about to be hurtful, by producing certain uneasiness, slight indeed at first, which as with regard to the lungs is a trifling sensation, and to the pores of the skin a kind of restlessness, which is difficult to describe, and few that feel it know the cause of it.
By eating moderately (as before advised for health's sake) less perspirable matter is produced in a given time; hence the bed-clothes receive it longer before they are saturated, and we may therefore sleep longer before we are made uneasy by their refusing to receive any more.
By using thinner and more porous bed-clothes, which will suffer the perspirable matter more easily to pass through them, we are less incommoded, such being longer tolerable.