Stress really can turn hair grey

A 2020 study in mice found the mechanism. Acute stress fires the sympathetic nerves that reach each hair follicle. They release noradrenaline, which pushes the melanocyte stem cells there to activate all at once. The stem cell pool runs out, and the new hair grows without pigment. The loss was permanent. Blocking the noradrenaline signal in the follicle prevented the greying.