Your turn
Each lab is one step from the video. Slide, tap and switch, and see what happens.
Each lab isolates one part of the story. The models are simplified, with illustrative numbers; they are not medical tools.
Tap the buttons and slide the sliders.
Slide the share of white hairs. What colour do your eyes see from far away?
In this simple lab each hair is either coloured or white; mixed together, the eye sees “gray”.
Slide it. More white hairs, and the hair looks lighter!
Switch the pigment cells off and on. Which part of the hair changes?
Only new growth at the bulb is affected; the keratinised shaft above keeps its colour.
Turn the colour helpers off. The new hair comes out white!
Drag the age slider. How do the new hairs change, cycle after cycle?
Drag age: fewer working melanocyte stem cells means less pigment in each new hair.
Slide from young to old. The new hairs get lighter.
Made-up, illustrative numbers to show a shrinking reserve. Not a measurement.
Push stress up, then ease it off. Which hair flips, and does it come back?
Rosenberg et al. (2021) saw some hairs lose and regain colour; their computer model suggests only hairs near a threshold can switch back and forth.
Illustrative, made-up numbers. In people, most gray hair is not caused by stress.
Decide first, then tap a card to check.
Use the follicle model to judge each claim, then reveal.
Guess, then tap a card!
Set age, genes and stress. Watch the salt and pepper appear.
Genes shift the timing, age advances it, and stress nudges only hairs near the edge.
Slide the sliders. Watch the white hairs appear one by one.
A simple illustrative model, not a prediction.
Gray hair is really a mix. Some hairs are dark and some are white. From far away, your eyes mix them into gray.
Every hair grows out of a tiny pocket in your skin. At the bottom live little colour helpers. They make a colour called melanin and paint each new hair as it grows.
Once a hair is out of your skin, its colour does not change. So a dark hair can't turn white. A new hair can grow in white.
New colour helpers come from a little store in the pocket. When people get older, the store gets smaller. In mice, scientists saw some store cells get stuck. Then new hairs grow in white, one by one.
Pull out a white hair and two grow back? Not true: one pocket grows one hair. A big scare turns hair white in one night? Not true. Worry is the main reason? Not really. Growing older, and your family, matter most.
Hair gets its colour from a pigment called melanin, made by pigment cells (melanocytes) at the bottom of each hair follicle.19 Those pigment cells are topped up from a small reserve of melanocyte stem cells in the follicle. As we age, that reserve slowly disappears, so new hairs grow in with little or no pigment.2 A white hair is simply a hair with no melanin, and a head of "gray" hair is mostly a mix of white and coloured hairs.12
Here's the surprise: a hair doesn't actually turn gray. Once a follicle makes a strand, its colour is set.8 Gray hair is new growth with little or no colour. Doctors call graying canities.7
Every hair grows from a hair follicle, a tiny tube-shaped pocket in your skin. More than 100,000 hairs grow on your head, each from its own follicle.16 At the bottom of each follicle is the bulb, where new cells are made. They push the hair up and out, at about 0.3 mm a day while it's growing.1
There are two kinds of melanin in hair. Eumelanin is brown-black and dominates in black and brown hair. Pheomelanin is yellow-red and is found more in auburn and blonde hair.15 More melanin means darker hair.9 Melanin is also what gives colour to your skin and eyes; for how your genes set these traits, see why DNA is important.
The hair you can see is dead, made of a tough protein called keratin. That's why a haircut doesn't hurt,1 and it's also why a strand that has already grown can't change colour.
Hair grows in cycles. A hair grows for about 2 to 7 years, has a short break of 2 to 3 weeks, rests for 2 to 4 months, and then a new hair grows and pushes the old one out.1 The American Academy of Dermatology says melanin is made fresh at the start of each cycle, and after about 7 to 15 cycles it stops.7
Each new hair needs fresh pigment cells, and these come from the stem-cell reserve. The NIH puts it simply: new melanocytes are made from melanocyte stem cells in the follicle; as we age, these stem cells gradually disappear, and hair that regrows without them has less pigment and looks gray.2 This pigment system wears out earlier than most other stem-cell systems in the body, which is why almost everyone goes gray eventually.4
"Stuck" stem cells. In 2023, a team at NYU Grossman School of Medicine filmed these stem cells in mice. They found the cells normally move, travelling between different parts of the follicle and switching between states as they go. With age, more and more of them got stranded in one place and stopped making pigment cells.4 The study was in mice, but it gives a clear picture of how the reserve can fail even when some stem cells are still there.
Hydrogen peroxide. Other researchers found that gray and white hairs build up hydrogen peroxide, and suggested it adds to oxidative damage involved in graying.13 Reviews say an imbalance in this kind of "oxidative stress" is likely part of the story, but warn against assuming mouse findings apply directly to people.14 Treat these as active research, not settled facts.
Why white looks white. Hair keratin without pigment is actually a pale yellowish colour. White hair looks white because light reflects off it, which hides that pale yellow.12 For more on hair follicles and the tiny muscles attached to them, see what causes goosebumps.
The age slider in the player uses made-up, illustrative numbers to show the idea of a shrinking reserve. It isn't a measurement.
When you get your first gray hairs, and how fully you go gray, is influenced mostly by the genes you inherit.8 Most people start seeing gray in their 30s or 40s, and many notice it around the same age their parents or grandparents did.711
In 2016, a UCL-led study of more than 6,000 people across Latin America found the first gene linked to graying: IRF4. This gene helps control how melanin is made and stored.6 It isn't the whole story, since many genes are likely involved, but it confirmed that graying has a genetic part and isn't only about the environment.
| What | Typical timing |
|---|---|
| First gray hairs (average) | 30s to 40s7 |
| "Premature" graying | before about 20 (light skin) or 30 (dark skin)9 |
| First gray to fully gray | often more than 10 years; some people never go fully gray911 |
| Where it happens | scalp, beard, eyebrows, eyelashes and body hair, not always at the same time9 |
Gray hair shows up more in dark hair because it stands out, but people with light hair are just as likely to go gray.11 Occasionally, early graying is linked to a health issue such as low vitamin B12 or thyroid disease, which a doctor can check.87
In mice, yes, and fast. In 2020, a Harvard team led by Ya-Chieh Hsu stressed mice and found they lost their pigment stem cells and went gray. The cause turned out not to be the immune system or stress hormones from the adrenal glands. It was the sympathetic nerves, the body's "fight or flight" wiring, which reach into every follicle. Under stress they released a burst of noradrenaline (also called norepinephrine). That made the resting stem cells all wake up at once, turn into pigment cells and move away, leaving none in reserve. "After just a few days, all of the melanocyte stem cells were lost," Hsu said.23
In people, it's less clear. Harvard Health notes that most human gray hair isn't related to stress, and it's unclear how often the mouse result applies to humans.8 Keeping your body's systems in balance matters in lots of ways; see why homeostasis is important.
Can gray hairs get their colour back? In 2021, a Columbia University team led by Martin Picard measured colour along single human hairs, which work a bit like tree rings: each hair keeps a record of the months it grew. They found some white or gray hairs that naturally regained colour. In one person, a hair lost its pigment when stress went up and got it back when stress went down. Their computer model suggests there's a threshold: only hairs that are close to turning gray anyway can flip back and forth.5 So stress may nudge a few hairs near the edge, but the model suggests easing stress is unlikely to bring back colour to hairs that are long past it. It isn't a treatment.
The stress graph in the player is illustrative, with made-up numbers, to show the threshold idea. It isn't real data.
No. Gray hair is a normal part of getting older, and there's nothing wrong with having it.7 It happens to everyone sooner or later, whatever their natural hair colour.911 You could even think of each strand as a little timeline of the months it grew, and of gray as one of the ways your body keeps time.5
Hair gets its colour from a pigment called melanin, made by pigment cells (melanocytes) at the bottom of each hair follicle. Those pigment cells are replaced from a small reserve of stem cells. As we get older, the reserve runs low or (in mice) gets stuck, so new hairs grow in with little or no melanin and look white. A head of 'gray' hair is mostly a mix of white and coloured hairs.
It can play a part, but it isn't the main cause. In a 2020 Harvard study, intense stress made mice go gray quickly: stress nerves released noradrenaline and used up the pigment stem cells in days. Whether this happens in people, and how often, is still unclear. For most people, genes and age matter far more.
Usually not. A hair's colour is set as it grows. But a 2021 Columbia University study measured colour along single hairs and found some gray hairs that naturally regained colour; in one person a hair lost colour when stress rose and regained it when stress fell. Their model suggests only hairs close to turning gray can flip. Doctors say there are no proven treatments to bring colour back, and gray hair is a normal part of ageing.
Mostly genes. People often notice their first gray hairs around the age their parents did, and a 2016 study linked a gene called IRF4, which helps control melanin, to graying. Doctors call graying 'premature' before about age 20 in people with light skin and before about 30 in people with dark skin. Occasionally a health problem such as low vitamin B12 or thyroid disease is involved, which a doctor can check.
No. Each follicle grows one hair, so plucking can't double them. The hair simply grows back gray, and dermatologists warn that plucking can thin your hair over time.
No. The hair you can see is already grown and made of dead cells, so it can't change colour. Stories of 'overnight' whitening are usually explained by a condition that makes coloured hairs fall out suddenly, so the white hairs that were already there suddenly stand out.
On average, people start to see gray hairs in their 30s or 40s, though some start much earlier or later. It can take more than 10 years to go from the first gray hair to a full head of gray, and some people never go fully gray.
This page explains how the body works; it isn't medical advice.