Why Do Hairs Go Gray Overnight?

Why does human hair go grey and is it ever possible for it to go white overnight from shock? Hannah and Adam explore why hair goes grey, how much stressful life events and a lack of sleep can speed up the process. They hear from the pilot whose hair turned white after a flight where all 4 of his engines failed after flying through a volcanic ash cloud - was the shock responsible? They also uncover new research which has shown it's possible for greying hair to return to its natural colour and ask if this finding could be exploited to uncover a cosmetic way to reverse hair greying?