We all have heard anecdotes of individuals whose hair turned white or gray due to stress. A group of Harvard researchers have discovered why that happens:

Stress activates nerves that are part of the fight-or-flight response, which in turn causes permanent damage to pigment-regenerating stem cells in hair follicles of mice. Their study on graying has just been published in Nature, and the results offer new insights into how stress can impact the body. For more info go to
https://news.harvard.edu/gazette/story/2020/01/new-findings-suggest-how-stress-may-trigger-gray-hair/