Can COVID -19 turn my hair grey overnight?

Actually, it is impossible for hair to turn grey overnight. A person might claim this happened during quarantine but it is more plausible that the graying of the hair either happened gradually during the 5 months of isolation or perhaps they simply did not have access to their usual hair dyes. The most famous example of hair supposedly turning grey overnight was that of Marie Antoinette, last queen of France. During the French Revolution she was put on trial and after her conviction sentenced to death by beheading. Folklore suggested that her hair turned grey overnight. A more logical explanation was the stress induced by her death sentence combined with a lengthy imprisonment led to the graying of her hair. Other possible logical explanations include her lose of access to hair dye or the confiscation of her wig to cover the grey hair. 

Hair color results from pigment cells located at the root of the hair under the skin. As hair sprouts, pigment cells add tiny drops of melanin to the hair at regular minute intervals to give hair its color. Once hair emerges from the scalp, it is dead and the pigment remains in the hair unless it is chemically bleached out. This process does not happen naturally.

If a person has both pigmented and un-pigmented hairs admixed, there is a process that can selectively attack the pigmented hairs. Known as Alopecia Areata, this autoimmune process can selectively attack the pigmented hairs causing those select hairs to fall out and the remaining grey hairs to be retained. This gives the allusion of hair turning grey over a short period of time. The hair would of course become thinner in this process.

Hair graying is a gradual process that is usually scattered over the entire scalp though not all hair follicles are affected simultaneously. Some hair follicles loose their ability to produce pigment while an adjacent follicle may retain this ability for a while longer. This gives the appearance of a salt and pepper hair. Totally graying happens gradually as intermittent dark hairs eventually become grey. Rarely, some people never turn completely grey. 

There are some diseases that cause temporary graying of the hair with the return to normal color once the primary process resolves. These include diseases or injury to the nervous system, endocrine disorders, physical shock, vitiligo and even influenza.

A recent study performed by a Harvard research team discovered that “when mice are stressed, their hair begins to grow gray due to a depletion in specific stem cells that reside in the base of hair follicles. These stem cells turn into pigment-producing cells as hair grows but in times of great stress these stem cells are overactivated. This depletes the follicle’s reservoir of pigment-producing stem cells, causing the subsequent hair growth to become gray.” “After just a few days, all of the pigment-regenerating stem cells were lost. Once they’re gone, you can’t regenerate pigments anymore. The damage is permanent.” 

https://newatlas.com/medical/harvard-how-stress-turns-hair-gray-stem-cells/