The greying of hair is a natural phenomenon — the pigment cells present in hair follicles that deliver melanin (color) to the strands of hair gradually die, leaving hair a transparent color like grey or white. While most commonly associated with aging, people can start sporting greying hair at any age. A new study explains how stress plays a role in this process.

Published in Nature, the study out of Harvard University and Harvard Stem Cell Institute shows stress activates the sympathetic nervous system, responsible for involuntary body processes such as heart rate, breathing, digestion, and what’s most relevant here — the fight-or-flight response.

While it has been known for almost a decade that stress influences the greying of hair, scientists didn’t understand how. Stress as a phenomenon affects all parts of the body, so a major obstacle for the latest study was to narrow down exactly which system affects the pigment production in hair follicles as a response to stress and how. Researchers first looked at the immune system, but quickly ruled it out — the hair of the mice they were experimenting on still turned grey even when the mice didn’t have immune cells; they then looked at the hormone cortisol, which the body produces in large amounts when experiencing stress — but the hair of the mice still turned grey even after researchers removed their adrenal glands that produce cortisol.

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Finally, they turned to the sympathetic nervous system, and bingo. They found the nerves in the sympathetic nervous system branch into the stem cells present in hair follicles. When the body is stressed, these nerves secrete the chemical norepinephrine, which is then absorbed by the pigment-producing stem cells inside the hair follicles. Norepinephrine excites these stem cells, thereby enabling them to produce hair pigments excessively, which quickly depletes their ability to produce pigments for a long time, essentially draining their pigment trove.

“After just a few days [of experiments on mice], all of the pigment-regenerating stem cells were lost. Once they’re gone, you can’t regenerate pigment anymore. The damage is permanent,” senior author of the study and the Alvin and Esta Star associate professor of stem cell and regenerative biology at Harvard, Ya-Chieh Hsu, said in a statement. “Acute stress, particularly the fight-or-flight response, has been traditionally viewed to be beneficial for an animal’s survival. But in this case, acute stress causes permanent depletion of stem cells,” lead author of the study, Bing Zhang, added.