Voyagers no longer embark in search of the storied Fountain of Youth, but the quest for longevity is still very much alive for researchers. Chronological age — the passing of time one spends on this planet — cannot be reversed, of course. However, biological age — one's health relative to that of one's peers — can be turned back. Healthy lifestyle habits contribute to "aging well," meaning one's biological age is younger than one's chronological age, researchers said. And sleep is a major factor in how well one ages.