Stress makes you get gray hair, we've all hear that before. However, we've never really know the exact reason, we just chalked it up to stress. Well, now we know, thanks to scientists at Harvard!
According to Time, they figured out and it has to do with your sympathetic nervous system. That's the system that's responsible for your "fight or flight" response under stress. It turns out that when it gets activated, it can produce a chemical that attacks the cells that color your hair.
So, when it happens enough times, your hair goes gray for good. And now you know!