If you see a picture of Obama before he came to the white house and then one as he left the white house, you would conclude that stress causes grey hair.
However, it is not that cut and dry. Or that black and white. It’s a bit of a grey area (pun intended).
A Harvard Medical School post from Robert H. Shmerling, MD states that “in humans, grey hair is not related to stress.”
…but it can trigger a common condition called telogen effluvium which causes hair to shed three times faster than normal.
Hair doesn’t actually “turn” grey, your hair follicles produce less color as they age. When your hair goes through its cycle of dying and regenerating, it is more likely to grow in as gray beginning after age 35.
So if your hair is falling out and regenerating more quickly because of stress, the hair that grows in could be more likely to be grey instead of its original color.
BUT there is a 2020 study of mice that shows stress can lead to their fur turning grey. Hair greying results from activation of the sympathetic nerves that innervate the melanocyte stem-cell niche. Under conditions of stress, the activation of these sympathetic nerves leads to burst release of the neurotransmitter noradrenaline (also known as norepinephrine).
Stress may be a trigger of grey hair.
Grey versus Gray