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Most of us have heard stories about someone “going grey overnight” after a shock or a brutal period of stress. Even though everyday experience suggested there was something real behind these stories, they often sounded more like drama than biology, because it was not clear how emotions could reach into a tiny hair follicle and change its colour.
Science. ¹
From greying anecdotes to mechanisms.
That picture is different now. A landmark study from Harvard University finally mapped the missing biological link. Researchers traced a direct chain from psychological stress, through the body’s stress-activated nerves, right into the stem cells that maintain hair pigment. This proved that stress acts as a physical "neuro-attack" on the follicle. While this attack can bankrupt the stem cell reserve, human mapping has since revealed that in earlier stages, the process can be dynamic — temporarily switching pigment production off and back on as stress levels change.
Why we need stress.
Stress is not just “feeling worried”. It is a built-in survival programme that briefly changes how the whole body works. The nervous system can run in two broad regimes:
The Sympathetic Nervous System.
The sympathetic “fight or flight” mode speeds up the heart, raises blood pressure and sends blood to the muscles to deal with danger. This is what mobilises the body.
The Parasympathetic Nervous System.
The parasympathetic “rest and digest” mode slows things down and supports digestion, repair and recovery.
In everyday life these two modes balance each other, but during stress the dial swings strongly toward the sympathetic side. When that happens, two stress responses are triggered:
“Fast stress” – nerve-driven fight or flight.
The sympathetic system switches on within seconds, releasing a burst of noradrenaline. Heart rate rises and blood is redirected to muscles to deal with danger. Crucially, these same nerves are wired directly to your hair follicles, making this "fast" response the key source of stem cell damage.
“Slow stress” – the hormonal HPA axis.
In parallel, the brain activates the HPA axis to release cortisol. This hormonal wave sustains the stress response for hours after the threat has passed. Research shows cortisol is not the primary enemy of pigmentation — more damage actually comes from the “fast” nerve response.
In short bursts these reactions are useful and protective. The problem starts when stress is strong or constant. Long, repeated activation gradually wears on tissues that depend on delicate long term balance. If stress keeps returning or never really stops, the HPA axis and sympathetic nerves are pushed again and again across the day. Hair follicles turn out to be one of the casualties.
How hair gets its colour and what has to break.
Hair gets its colour from melanocytes, specialised cells in each follicle that pack melanin pigment into the growing hair shaft. These melanocytes do not last forever. They are constantly replaced from a small reserve of melanocyte stem cells (MSCs) that live in a protected niche in the bulge region of the follicle.
Each follicle cycles through growth (anagen), regression (catagen) and rest (telogen). At the start of a new growth phase, some MSCs “wake up”, leave the niche and turn into pigment cells that colour the new hair. Others stay behind to keep the reserve going. That reserve is finite and largely set early in life. As the years pass, damage and normal wear slowly reduce both the number and the quality of these stem cells. Once the pool for a given follicle is exhausted or too damaged to work, that follicle can no longer make enough pigment. From that point on, the hairs it produces grow in grey or white.
This delicate renewal process is not isolated from the rest of the body. It is wired into the same sympathetic nerves that drive the stress response, which reach into the bulge niche and signal directly to MSCs. What happens when those nerves fire under stress is the crucial step that links feeling under pressure to hair turning grey.
Stress and the collapse of pigment reserves.
Hair follicles are wired to the stress system.
The bulge region that houses melanocyte stem cells is not a closed, isolated pocket. It is tightly wrapped in sympathetic nerve fibres, the same branch of the nervous system that drives the fight-or-flight response. When these nerves fire, they release noradrenaline directly into the stem cell niche, putting the pigment reservoir under the direct influence of stress signalling.
Acute intense stress floods the niche with noradrenaline.
In the Nature mouse experiments, animals were exposed to intense stressors that massively activated the sympathetic fibres around hair follicles [Ref 1]. Because the mice turned grey even without adrenal glands or immune cells, researchers determined that this specific depletion operates independently of cortisol or immune attacks. Instead, the study showed that stress acts directly through the “fight-or-flight” nerves, which release noradrenaline into the MSC niche and directly deplete the pigment-regenerating stem cells [Ref 2].
The key culprit is noradrenaline released from sympathetic “fight-or-flight” nerves directly into the pigment stem-cell niche.
Noradrenaline pushes stem cells into destructive overdrive.
Under this surge of noradrenaline, normally quiet MSCs are jolted out of their resting state. Instead of pacing their activity over many hair cycles, they start to proliferate and differentiate almost all at once. A large fraction of the stem cells convert into melanocytes and migrate out of the bulge to join the pigment-producing compartment. In the process, the niche loses its long-term reserve of stem cells in a very short time.
Loss of pigment capacity.
Once those MSCs are gone, the body does not repopulate that niche for that particular follicle. The next hair cycle begins with no pigment-producing cells left to call on. New hairs from those follicles emerge grey or white and remain that way in subsequent cycles. In this model, severe stress does not bleach the hair that is already on the head. It spends the stem-cell savings account in one shot, sacrificing future pigment capacity to a single extreme stress event.
Interesting fact
Evolutionary gain of grey hair.
If the body has a built-in way to lose hair colour under stress, it is reasonable to ask whether this ever brought an evolutionary benefit. One idea is that stress-induced greying acts as a visible status signal [Ref 3]. Because grey hair is strongly associated with age, it can serve as a quick cue for experience, competence and leadership. In mountain gorillas, for example, fully mature males develop a silver back and often go on to lead a troop [Ref 3]. By analogy, an individual who has survived enough challenges to “earn” grey hair might be seen as more seasoned and trustworthy than their age alone would suggest.
Human evidence and reversibility.
It is still uncertain whether real-world stress always pushes follicles all the way to permanent loss, or whether it can sometimes only switch pigment production into an off state. A separate human study that followed colour bands along single hairs found that some grey hairs darkened again when everyday stress eased [Ref 4]. This suggests that in at least some follicles the pigment system is not fully destroyed but can be switched back on, hinting at partial reversibility.
KEY SCIENTIFIC REFERENCES.
- Zhang, B., Ma, S., Rachmin, I., He, M., Baral, P., Choi, S., et al. (2020). Hyperactivation of sympathetic nerves drives depletion of melanocyte stem cells. Nature.View Publication
- Harvard Stem Cell Institute. (2020). Solving a biological puzzle: How stress causes gray hair. News article.View Publication
- Clark, S. A., & Deppmann, C. D. (2020). How the stress of fight or flight turns hair white. Nature.View Publication
- Rosenberg, A. M., Rausser, S., Ren, J., Mosharov, E. V., Sturm, G., Ogden, R. T., et al. (2021). Quantitative mapping of human hair greying and reversal in relation to life stress. eLife.View Publication