On the Making of Man by Gregory of Nyssa — Read

This life of our bodies, material and subject to flux, always advancing by way of motion, finds the power of its being in this, that it never rests from its motion: and as some river, flowing on by its own impulse, keeps the channel in which it runs well filled, yet is not seen in the same water always at the same place, but part of it glides away while part comes flowing on, so, too, the material element of our life here suffers change in the continuity of its succession of opposites by way of motion and flux, so that it never can desist from change, but in its inability to rest keeps up unceasingly its motion alternating by like ways: and if it should ever cease moving it will assuredly have cessation also of its being.

For instance, emptying succeeds fullness, and on the other hand after emptiness comes in turn a process of filling: sleep relaxes the strain of waking, and, again, awakening braces up what had become slack: and neither of these abides continually, but both give way, each at the other's coming; nature thus by their interchange so renewing herself as, while partaking of each in turn, to pass from the one to the other without break. For that the living creature should always be exerting itself in its operations produces a certain rupture and severance of the overstrained part; and continual quiescence of the body brings about a certain dissolution and laxity in its frame: but to be in touch with each of these at the proper times in a moderate degree is a staying-power of nature, which, by continual transference to the opposed states, gives herself in each of them rest from the other. Thus she finds the body on the strain through wakefulness, and devises relaxation for the strain by means of sleep, giving the perceptive faculties rest for the time from their operations, loosing them like horses from the chariots after the race.