For the longest time now, scientists have been debating whether or not stress can cause graying of hair. Finally, a study by Harvard University has found this hypothesis to be true. The researchers have discovered that indeed, stress activates the nerves that are part of fight or flight response. This causes permanent damage to pigment- generating stem cells in hair follicles.
The color of your hair is determined by cells called melanocytes. These cells produce the pigment melanin. New melanocytes are made from melanocyte stem cells, which are found within the hair follicle, specifically at the base. Aging causes these cells to disappear gradually. This means that no more melanocytes will be produced and hence no melanin. The hair that will therefore grow from your follicles will have less melanocytes, making it appear gray.
In this research, a link between graying of hair and stress was found. A mice was used due to its numerous similarities to human beings in terms of anatomy, physiology and genetics. Sympathetic nerves (whose system is responsible for the fight or flight response) branch out into each hair follicle on the skin. Stress causes these nerves to release norepinephrine chemical which absorbed by the nearby pigment generating stem cells. This therefore, is responsible for the change of stem cells into melanocytes and migrating away from their reserve, at the base of the hair strand. With no more stem cells, no new pigment cells can be made and any new hair becomes gray then white.