There is a belief that a shock or a state of extreme stress can cause hair to turn grey overnight. At first glance, there are plausible explanations for this phenomenon, but is it really true that stress causes grey hair?

It has been proven that there are biological mechanisms in the body that influence hair growth during emotional stress. Human hair grows in cycles. During the anagen phase, the hair grows and pigments are produced in the hair follicles. This phase is followed by the catagen phase, the resting phase. The telogen phase completes the cycle and is an inactive phase.
The duration of the anagen phase is genetically determined and is also influenced by hormones. It usually lasts between two and eight years. At the end of the anagen phase, the follicles stop producing pigment and prepare for the telogen phase. This lasts between six months and one and a half years until the anagen phase begins again.
This cycle is repeated around ten times before the follicles stop producing pigment and colourless hair grows. We refer to this pigmentless hair as grey or white hair.

Severe stress can cause an increased number of follicles to enter the telogen phase at the same time. This phenomenon is medically referred to as telogen effluvium and is often caused by medications that affect the hormonal control of the hair cycle.

However, the sudden, simultaneous entry of many follicles into the telogen phase does not happen overnight. The reason why it often gives the impression that grey hair appears suddenly lies in people's perception and memory. People are more likely to remember the stressed people who have gone grey than the stressed people who go through the difficult phase without any visible greying. People also tend to notice people with premature greying who have had no stress at all.

Stress and shock can therefore cause the follicles to stop producing pigment prematurely, but this does not happen from one day to the next.