When mice are stressed, their hair turns gray because of nerves, not because of hormones from their adrenal glands. Cutting out the glands doesn’t stop the greying, but turning off the nerves does.

When mice experience sudden stress, their nerves send a signal that causes a specific type of stem cell in their hair follicles to multiply and then disappear, making their fur turn gray permanently — and this happens without involving their hormones or immune system.

When mice are stressed, their hair turns gray because a chemical called norepinephrine activates a specific receptor on hair color cells. If you block that receptor, their hair doesn't turn gray—even when stressed. And if you inject the chemical directly, their hair turns gray even without stress.

When mice experience sudden stress, it causes the cells that give hair its color to panic and leave their home in the hair follicle, burning out too soon and making the fur turn gray forever.

In mice with a specific genetic problem, hair turns gray because the stem cells that make hair color die off first—not because the color-making process itself is broken.

Stress can turn your hair gray by messing with nerve signals in your hair follicles, which damages the cells that give hair its color — and once they're gone, the graying is permanent.