It may sound like an old wives' tale, but stress really could make you go grey.
Legend has it Marie Antoinette went grey overnight before her beheading in 1791. We now know this to be fantastical, as the shaft of hair will not lose its colour just because someone is on edge.
Stress may affect the hue of hair coming out of the follicles, however.
Scientists from the Columbia University Vagelos College of Physicians and Surgeons analysed tiny slices – around 0.2mm – of 14 people's hair, representing about one hour's growth.
After comparing the slices against the participants' stress diaries, the team found "striking associations" between greying and feeling frazzled.
In more positive news, combating the stress was found to somewhat reverse the greying process.
"Just as the rings in a tree trunk hold information about past decades in the life of a tree, our hair contains information about our biological history," said study author Dr Martin Picard.
"When hairs are still under the skin as follicles, they are subject to the influence of stress hormones and other things happening in our mind and body.
"Once hairs grow out of the scalp, they harden and permanently crystallise these exposures into a stable form."
Stress has long been linked to grey hair, however, experts have questioned whether this held any scientific basis.
To learn more, the Columbia scientists developed a new method of capturing highly detailed images of tiny human hair slices.
"If you use your eyes to look at a hair, it will seem like it's the same colour throughout unless there is a major transition," said Dr Picard.
"Under a high-resolution scanner, you see small, subtle variations in colour – and that's what we're measuring."
After providing hair slice samples throughout a year, the 14 participants rated when they were most stressed over the 12 months.
The scientists noted a robust link between the participants' frazzled periods and when grey hair grew out of their follicles.
Some of the hairs regained their original colour when the stressful phase passed, however.
"There was one individual who went on vacation and five hairs on that person's head reverted back to dark during the vacation, synchronised in time," said Dr Picard.
This contradicts a recent mouse study that suggests stress-induced grey hairs are permanent.
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To better understand how stress is linked to greying, the scientists also measured thousands of proteins in the participants' hair.
Results, published in the journal eLife, reveal 300 proteins changed their expression alongside the onset of grey hair.
This is thought to come down to stress-induced variations in mitochondria, the "energy powerhouses" of cells.
"We often hear the mitochondria are the powerhouses of the cell, but that's not the only role they play," said Dr Picard.
"Mitochondria are actually like little antennas inside the cell that respond to a number of different signals, including psychological stress."
This may explain why the previous study found greying was irreversible in mice.
"Mice have very different hair follicle biology and this may be an instance where findings in mice don't translate well to people", said co-author Dr Ralf Paus, from the University of Miami Miller School of Medicine.
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The scientists hope their results will allow other experts to get a better handle on ageing.
"Understanding the mechanisms that allow 'old' grey hairs to return to their 'young' pigmented states could yield new clues about the malleability of human ageing in general and how it is influenced by stress," said Dr Picard.
"Our data add to a growing body of evidence demonstrating that human ageing is not a linear, fixed biological process, but may – at least in part – be halted or even temporarily reversed."
While we should all be working towards a less stressful existence, a zen lifestyle is unlikely to completely restore grey hair to its former hue.
"Based on our mathematical modelling, we think hair needs to reach a threshold before it turns grey," said Dr Picard.
"In middle age, when the hair is near that threshold because of biological age and other factors, stress will push it over the threshold and it transitions to grey.
"We don't think reducing stress in a 70-year-old who's been grey for years will darken their hair or increasing stress in a 10-year-old will be enough to tip their hair over the grey threshold."