Marie Antoinette's hair allegedly went white the night before she was executed. Claudia Hammond explores whether it's scientifically possible:
The medical name for the sudden whitening of the hair is canities subita. The best explanation for it is not that the hair changes colour, but that the pigmented hairs fall out. A condition called alopecia ariata causes the hair shed suddenly, resulting in bald patches. It is thought to be caused by an auto-immune response, where the body’s defence system turns on itself. It can be exacerbated by stress, which could account for the association of white hair with terrifying experiences. In some cases the white hairs are unaffected by the condition. So a severe shock could trigger hair loss, but with only the coloured hairs falling out, leaving someone who already has some grey with a whole head of white hair.
Update from a reader:
Anecdotal evidence: the night my wife went into (very) pre-term labor with our twins was off-the-charts stressful for everyone involved. My beard started coming in salt-and-pepper the next day. 100% grey? Nope. But a dramatic change? You bet.
Almost 13 years ago I was walking home from work and I was hit by a car. I went into cardiac arrest, had an epidural hematoma and was put into a drug induced coma for 21 days. Before the accident I had only a few strands of gray hair. When I woke out of the coma about a 1/3 of my hair was gray.