Jonah Lehrer explains them:

Here's one example of how stress destroys the body. Elissa Epel, a former grad student of Sapolsky's and a professor of psychiatry at UCSF, has demonstrated that mothers caring for chronically ill report much higher levels of stress. That's not surprising. What is surprising is that these women also have dramatically shortened telomeres, those caps on the end of chromosomes that keep our DNA from disintegrating. (Women with the highest levels of stress had telomere shortening equal "to at least one decade of additional aging.") When our telomeres run out, our cells stop dividing; we've run out of life. Stress makes us run out of life faster.

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