Can stress turn your hair gray? In mice, the answer is yes. Hair gets its color from special cells. These cells come from stem cells near the root of the hair.

When mice were under strong stress, their nerves became very active. The stem cells were all used up, so new hair grew white. In humans, some gray hairs even got their color back when stress went down.

Stress is widely thought to cause gray hair, and research in mice supports this idea. Hair color comes from pigment cells, which are supplied by stem cells near the hair root. In 2020, a team at Harvard found that stressed mice grew many white hairs. The cause was not stress hormones.

Graying still happened when the glands that make these hormones were removed, but it did not happen when sympathetic nerves were blocked. These nerves, which prepare the body to fight or run away, released a chemical that made all the stem cells activate at once and run out. In 2021, researchers studying human hair found that some gray hairs regained color during periods of lower stress.

In 2020, a Harvard team put mice through stressors such as pain and restraint and then watched white hairs appear in the fur that grew back. Hair color depends on melanocytes, pigment cells that are replenished from a reservoir of melanocyte stem cells in the follicle. Earlier work by Nishimura and colleagues had shown that graying with age reflects a gradual failure to maintain these stem cells, and that DNA damage can force them to mature before they renew themselves.

The mice still went gray after their adrenal glands were removed and when they lacked immune function, but blocking sympathetic nerve signals prevented it. Under stress, those nerves released noradrenaline. Dormant stem cells began to multiply and differentiate until the reservoir was used up for good. Human evidence is thinner, and it has not been shown whether the same nerve pathway operates in people.

In 2021, Columbia researchers measured pigment along single hairs, which grow about one centimeter a month. Their observational study was small, and the hairs were probably close to turning gray anyway. Even so, they found gray hairs that had regained their color. In a few participants, graying lined up with stressful periods and recovery with calmer ones.