- Video report by ITV News correspondent Rebecca Barry
Scientists believe they have uncovered "new evidence" that shows acute stress can cause hair to turn white prematurely.
In experiments conducted on mice, scientists found that extreme levels of stress caused the sympathetic nervous system to become hyperactive.
This in turn depletes the the number of stem cells responsible for hair colour and so causes pigment loss and the development of white hair.
The team behind the research said their findings contradict previous theories that stress-related grey hair is a result of immune attacks or stress hormones such as cortisol.
The research, published in the journal Nature, gives clarity to previous studies that linked stress to premature and accelerated hair greying without explaining the mechanism behind the link.
Dr Ya-Chieh Hsu, a professor of regenerative biology at Harvard University in Massachusetts, US, and a senior author on the study, said his work lays "the groundwork for understanding how stress affects other tissues and organs in the body".
The researchers say their findings do not provide a cure or treatment for grey hair but give an idea of how stress might affect many other parts of the body.
Dr Hsu said: "By understanding precisely how stress affects stem cells that regenerate pigment, we've laid the groundwork for understanding how stress affects other tissues and organs in the body.
"Understanding how our tissues change under stress is the first critical step towards eventual treatment that can halt or revert the detrimental impact of stress.
He added: "We still have a lot to learn in this area."
Hair colour is determined by cells called melanocytes, which are derived from melanocyte stem cells (MeSCs).
As people age, the supply of MeSCs is gradually depleted, causing pigmented hair to be replaced with white hair.
Researchers at Harvard were studying the effects of pain on mice by injecting a toxin when they discovered the rodents' fur turned white in just four weeks.
This prompted the Harvard team to collaborate with scientists around the world to investigate the biological mechanism that led to the drastic change in the rodents' hair colour.