Hair naturally turns grey as we age because the pigment cells in hair follicles that give hair its color start to die off. Without enough melanin, hair takes on a grey or white hue. However, people can go grey at any age, and now, new research sheds light on how stress speeds up this process. This research, published in Nature by Harvard University and the Harvard Stem Cell Institute, found that stress triggers the sympathetic nervous system, which controls automatic functions like digestion, heart rate, and breathing.

More importantly, it kicks in the “fight-or-flight” response, which seems to be linked to greying hair. For years, Scientists have known that stress can make hair turn grey, but they didn’t fully understand the mechanism behind it. Stress affects many parts of the body, making it difficult to figure out how it impacts hair pigment production specifically.

In this study, researchers first thought the immune system might be the cause, but even without immune cells, the mice in the experiment still turned grey. Then, they looked at the hormone cortisol, which spikes during stress. However, when they removed the mice’s adrenal glands (which produce cortisol), their hair still turned grey, ruling out that theory as well.

The researchers eventually focused on the sympathetic nervous system, and that’s where they found the answer. Nerves from this system reach into the stem cells of hair follicles. When the body experiences stress, these nerves release norepinephrine, a chemical that gets absorbed by the pigment-producing stem cells, this chemical over-stimulates the cells causing them to produce pigment in excess. In response, by working overtime, the stem cells burn quickly leaving them unable to keep their ability to keep making hair color over time, leading to premature greying.

In the study, after just a few days of stress experiments on mice, the pigment-regenerating stem cells were completely depleted. Ya-Chieh Hsu, senior author of the study and a professor of stem cell biology at Harvard explained that once those cells are gone, there’s no way to regenerate the pigment, meaning the greying will become permanent. While the fight-or-flight response is usually helpful for survival, in this case, it causes permanent loss of the stem cells that produce hair pigment.