Can hair turn white overnight?

Several legends recount how certain historical figures saw their hair turn white overnight. This phenomenon, known as sudden canities, was first mentioned in the 6th century BC. It is said that the hair and beard of the Chinese warrior Wu ZIXU turned white in less than a week when his family and he were disgraced by King Ping of Chu and he found himself the sole survivor.

One might also think of the Queen of France Marie-Antoinette and the humanist Thomas MORE, author of Utopia. It is said that both of them saw their hair "turn white from misfortune" the night before their executions. Finally, the French actor Jean GABIN reported that his hair turned white the night before his military convoy was attacked during the Algerian War. These various episodes are debated by historians and do not have a global consensus.

Typically, the appearance of white hair is gradual and often begins in the late thirties. This is due to the decrease in activity of the melanocytes and the reduction in their number as we age. These cells are responsible for the synthesis of melanin, the pigment that gives hair its colour. In the absence of melanin, the hair turns white. Other factors can speed up or slow down the whitening of hair, such as stress, exposure to UV rays, pollution or tobacco, certain autoimmune diseases, genetics or even diet.

As a reminder, the process of hair growth occurs in cycles that include several phases. The first is the anagen phase (growth phase) which lasts from 2 to 7 years. This is followed by the catagen phase (resting phase) which extends from 2 to 3 weeks. Finally, the cycle concludes with the telogen phase, lasting approximately 3 months, during which the hair detaches, pushed out by a new developing hair bulb. During the anagen phase, melanin is produced by the melanocytes located at the base of the follicle and transferred to the cells of the growing hair. The hair is thus pigmented when the melanocytes are active.

When considering the mechanisms of hair growth and pigmentation, and the duration of a follicular cycle, it initially appears difficult for hair to turn white overnight. Scientists have explored this paradox in various studies and the following hypothesis has been accepted as the most likely: it would be an autoimmune reaction, triggered by an emotional shock. This is actually a specific case of alopecia, or alopecia areata. The immune system mistakenly attacks the hair follicles, leading to inflammation. The hair growth cycle is then disrupted and the hair becomes weaker. Eventually, they break and end up falling out.

In the case of sudden greying, the targeted follicles would be only those producing coloured hair. In the case of hair that was originally salt and pepper, only the white hairs would remain, creating an optical effect and the impression that the hair has completely turned white. However, this is a phenomenon that is extremely rare and still debated within the scientific community. Thus, even though it has been proven that stress accelerates greying, rest assured: it is highly unlikely that your hair will turn white overnight, even in cases of intense stress.

• LEVELL N. & al. Sudden whitening of the hair: an historical fiction? Journal of the Royal Society of Medicine (2008).

• RAWNSLEY J. & al. Hair biology: Growth and pigmentation. Facial Plastic Surgery Clinics of North America (2018).

• PHILPOTT M. Watching hair turn grey. eLife (2021).