re: How fast can hair go grey?

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Overnight...

Canities subita, also called Marie Antoinette syndrome or Thomas More syndrome, is an alleged condition of hair turning white overnight due to stress or trauma.[1] The trivial names come from specific cases in history including that of Queen Marie Antoinette of France whose hair was noted as having turned stark white overnight after her capture following the ill-fated flight to Varennes during the French Revolution. An older case of Sir Thomas More's hair turning white the night before his beheading has also been recorded. Although a number of cases of rapid hair greying have been documented, the underlying patho-physiological changes have not been sufficiently studied.

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In humans, most gray hair is not related to stress. In fact, hair doesn't actually "turn" gray at all. Once a hair follicle produces hair, the color is set. If a single strand of hair starts out brown (or red or black or blond), it is never going to change its color (unless you color your hair). Your hair follicles produce less color as they age, so when hair goes through its natural cycle of dying and being regenerated, it's more likely to grow in as gray beginning after age 35. Genetics can play a role in when this starts.

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