Arianna Huffington: We don't yet know what big challenge Hillary Clinton will take on next, but that she will bring her considerable talents to something big and worthwhile is not in question. Whether or not that challenge is trying to become the first woman president, she's in a unique position to help redefine success by using her experience to address the issue of stress in the workplace. Lack of sleep, overwork, and burnout are defining features of America's business and political culture. Hillary Clinton is in a singular position to change this. As she said in November, "I would like to see whether I can get untired." And maybe I'm dreaming, but the world needs Hillary not only to get herself "untired," but in the next chapter of her life to become a role model for the idea that one can both be untired and successful. If so, there's no ceiling on what she could accomplish for women -- and yes, for men too.