Is hair greying permanent?

Biology offered a clear explanation for this. Inside each hair follicle there is a small reservoir of melanocyte stem cells. These cells act like a pigment seed bank. They replenish the pigment-making cells that load melanin into the growing hair shaft and give it colour.

With age and repeated stress, this seed bank is thought to shrink. Fewer stem cells are left to replace worn-out pigment cells. At some point the reservoir is so depleted that the follicle can no longer supply enough melanin. The hair that grows out from that follicle emerges pale, and then fully grey or white.

In this classic model, once the stem-cell bank is effectively empty, the process is considered irreversible. That follicle has crossed a line. It is expected to keep producing grey hair for the rest of its life, which is why greying has been framed as a one-way road rather than something the body can meaningfully reverse.

In the last few years, however, new work has started to challenge this simple picture. In particular, a 2021 study led by Ayelet Rosenberg at Columbia suggests a more nuanced scheme of hair greying, where at least some follicles retain the capacity to switch pigment production off and on again under certain conditions [Ref 1].

To say anything meaningful about greying, you first need a reliable method to measure it. The Columbia study provided one by treating each hair as a timeline of pigment change.

Scalp hair grows at a fairly steady pace, roughly 1–1.3 centimetres per month, so each point along a single hair corresponds to a particular week in the past. The tip reflects months ago; the root reflects what is happening now. Like growth rings in a tree trunk, hair length becomes a physical record of time.

Instead of simply calling a hair grey or not grey, the researchers scanned the shaft using high-resolution imaging and converted its colour into numerical values along the entire length. This produced detailed hair-pigmentation patterns showing where the strand is darker, lighter, or fully depigmented, millimetre by millimetre.

With this method, a single hair becomes a high-resolution record of how pigmentation changes in one follicle over time. The team could see when colour began to fade, when it stabilised, and — unexpectedly — when it returned, all mapped onto an actual time axis rather than reduced to a before-and-after snapshot.

This level of detail matters because greying is not a centralised process. Each follicle behaves autonomously, and transitions from one phase to the next happen independently in each follicle [Ref 2]. High-resolution mapping along single hairs is what makes those individual follicle shifts visible.

When the researchers applied this new mapping, they found that some hairs did not simply progress from dark to grey — they became darker again after greying.

They had expected to document the usual one-way march from dark to grey. Instead they found something stranger. In several volunteers, some strands were bi-colour, or even multi-phase, with sharp transitions in pigmentation along the same hair.

In a few cases the pattern was simple: a dark segment near the tip faded into a pale or fully grey segment closer to the root, and then stayed grey — the classic story. But other hairs showed the opposite twist. They started dark, turned grey for a stretch, and then became darker again toward the root.

The study found that shifts in hair colour line up closely with shifts in stress.

To test that link, the researchers asked volunteers to reconstruct their stress levels over the previous months. People went through calendars and noted periods of intense pressure at work, relationship problems, or major life changes. This produced a personal stress timeline for each participant.

Because hair length maps onto time, the team could line up these stress timelines with the pigmentation patterns along each strand. In several striking cases the match was tight. A hair segment turned grey during a clearly stressful period, then returned to darker pigment in the weeks after the stress eased. The same hair recorded both a stress-linked loss of colour and a recovery when conditions improved.

The group also looked inside dark and grey hairs using proteomics. If the classic story of simple depletion were fully correct, one might expect grey hairs to be empty or inert. Instead they were packed with proteins, and not at random. Grey hairs showed higher levels of proteins linked to mitochondria, energy metabolism, and stress responses, alongside changes in structural and pigment-handling proteins.

This points to a different picture. Many grey hairs are not dead, pigmentless leftovers. They are remodelled tissues that have shifted into a high-stress, altered-metabolism state [Ref 1]. In that state, pigment production is dialled down or shut off. Under the right conditions some follicles can move out of that state again and restart pigment — which fits with the rare but real reversals seen in the hair timelines.

The study suggests that greying can remain reversible while a follicle is still close to a biological threshold. To explain this, the authors propose a threshold model for each follicle.

Every hair is imagined as carrying an internal load that reflects both underlying ageing processes and short-term stress. This load is not fixed. It creeps up over the years as damage and wear accumulate, jumps during stressful periods, and can ease back a little when conditions improve.

In this picture, visible greying happens when that combined ageing-and-stress load crosses a critical threshold. Once a follicle is pushed over the line, pigment production drops, and the hair segment that grows during that time comes out grey or white.

The key nuance is what happens to follicles that sit close to the threshold, rather than far beyond it. If a hair is just over the line, reducing stress or improving the local environment can pull the load back down. In those borderline follicles the pigment machinery can restart — which shows up as a grey segment followed by a darker segment on the same strand.

This framework explains several observations at once. Reversals are rare because many follicles are either comfortably below the threshold and still pigmented, or far above it and stably grey. They are more likely in younger or early-greying individuals, where many follicles hover near the tipping point, and usually affect only a small fraction of hairs. Recent work suggests greying remains reversible while melanocyte stem cells and follicular melanocytes are still present [Ref 3]; follicles that have not completely lost these cells may regain pigment if their microenvironment and energy balance recover.