Why hair turns gray
From Wigipedia, the free hair encyclopedia
Each hair grows from a follicle that comes with its own paint shop: pigment cells called melanocytes that load the growing strand with melanin, the pigment that also colors skin and eyes. As we age, those cells become fewer and less productive. New hair grows in with less pigment, then none. The strand goes white.[citation needed]
There is no gray pigment
The single most surprising thing about gray hair is that there is no such thing as a gray hair. Individual strands are either pigmented or white. “Gray” is what your eye reports when it averages a head carrying both at once. A “salt and pepper” beard is exactly that: salt and pepper, not a third blended seasoning.
There is also a chemistry angle. Follicles naturally produce a little hydrogen peroxide, and young follicles mop it up with an enzyme called catalase. As catalase declines with age, the peroxide accumulates and effectively bleaches the hair from the inside — so hair is not just losing color, it is being mildly de-colored by its own plumbing.
The overnight myth
Folklore is full of people whose hair turned white overnight from terror or grief. The most famous is Marie Antoinette, whose hair was said to have whitened the night before her execution in 1793 — the legend even lent its name to Marie Antoinette syndrome.
The trouble is arithmetic again: the visible part of a hair is dead, finished material, and nothing that happens to your nerves can repaint it. A strand that grew in brown stays brown. What can happen quickly is a sudden, patchy loss of the pigmented hairs (as in some forms of the autoimmune condition alopecia areata), leaving the already-white ones behind and creating the impression of overnight change. Fast, yes. Overnight repainting, no.
Does stress count?
A little. A 2020 study found that intense stress can drive the sympathetic nervous system to burn through the follicle’s melanocyte stem cells, and — at least in mice — some of the change reversed when the stress stopped. So stress can nudge the timeline. Your genes and your birthday still do most of the work.