I see the healthy repeatedly follow a powerful (but nebulous) rule that the unhealthy ignore. William James hints at the rule in the following paragraph (from his book The Principles of Psychology):

"Keep the faculty of effort alive in you by a little gratuitous exercise every day. That is be systematically ascetic or heroic in little unnecessary points, do every day or two something for no other reason than that you would rather not do it, so that when the hour of dire need draws nigh, it may find you not unnerved and untrained to stand the test."