For centuries there have been reports of people who suddenly have hair turn white from fright or from tremendously stressful circumstances. According to many accounts, Marie Antoinette had her hair turn white the night before she was executed. Although this makes for a more dramatic story, evidence suggests Marie Antoinette's hair turned white far prior to this, and the process was not sudden.
The trouble with all accounts of people suddenly experiencing their hair turn white is that it’s not possible. White or gray hair begins at the hair shaft, so hair on your head is already dead material. It can’t suddenly change color, unless you visit a local beauty salon. Newly white hair must grow from the roots out, at the rate of typical hair growth, making the overnight process of having one’s hair turn white from fright an unlikely scenario.
Still, there are many who insist that people have had their hair turn white in an overnight situation of extreme stress. It is possible for it to appear as though a person suddenly woke up with white hair. This process requires that the person already have large amounts of white or gray hair before the change.
What can happen to hair in highly stressful circumstances is that some hair might fall out, in a condition called diffuse alopecia areata. This condition results in significant hair loss and occurs rapidly. In these instances, hair that is not colored could fall out, while some white hair or grey hair remains. This would give the appearance that one had their hair turn white overnight. In fact, what really occurred is someone lost a great deal of colored hair in a short period of time.
Alopecia areata tends not to occur overnight, but can occur suddenly. It tends to be linked to autoimmune disease where the cells in the body suddenly view the hair cells as foreign matter and attack them.
The result can be patches of baldness on the head, and in worst cases, all over the body. Normally alopecia areata is treated with medications like prednisone, which allows the body to shut down its immune response. Treatments with prednisone was unavailable until the 20th century, perhaps accounting for more "white overnight" stories earlier.
It’s unclear what triggers alopecia areata, but it can occur in men, women and children. In people with an abundance of gray hair, this could cause one to appear as though the hair had suddenly turned white. It would also result in patchy baldness, so the hair would appear thin. Other causes of sudden baldness include bad cases of ringworm, not terribly uncommon in previous centuries, or severe hyperthyroidism, which can cause sudden and significant hair loss.
Accounts of those who have their hair turn white suddenly are essentially urban legends. It’s more reasonable to say that if hair suddenly appeared white, it was due to significant loss of colored hair.