"An instant gray hair": whether can such occur actually

The decoloured hairs start appearing at people of any floor already aged is more senior than 25 years. Gradually, by 50 years, at the person some sites of the head, for example, whisky, a nape, a bang turn gray, or it completely becomes whitish. There is an opinion that the individual because of a stress can instantly turn gray, and in some hours. But whether there is such actually and, if yes, that why? In world history there will be many stories how people instantly changed in old men and their hair from the naturally dark turned into the gray-haired. Contemporaries of the English writer narrate about the similar...