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How does a hair follicle ‘know’ that a hair has been cut? © Getty Images

How does a hair follicle ‘know’ that a hair has been cut?

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In short, it doesn't, but getting your hair cut can have impacts on how much you see your hair grow and thicken.

Asked by: D J Gill, Beccles

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It doesn’t. The hairs on your head grow for between two and seven years. This lifespan is controlled by your genes and is the limiting factor for your hair’s maximum length. Hair is a dead protein structure built at a more or less constant rate by the cellular machinery within the follicle, and the weight of the hair above it doesn’t apply any ‘braking force’ to slow it down. If you cut your hair very short, it may appear to grow quicker for a while, but this is just because a month’s growth represents a larger percentage increase on short hair than on long hair.

Once follicles reach the end of their growing (or anagen) phase, they spend a few weeks in the catagen phase while the hair root is broken down. That hair then falls out and after three months in the telogen, or resting, phase a new hair begins to grow. At any given point, 90 per cent of your hairs are anagenic, one per cent catagenic and nine per cent telogenic. If you shave your skin smooth, the anagenic hairs that continue to grow will be joined by the telogenic hairs emerging for the first time, and your hair will appear slightly thicker for a while until the catagenic hairs start falling out again.


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Authors

luis villazon
Luis VillazonQ&A expert

Luis trained as a zoologist, but now works as a science and technology educator. In his spare time he builds 3D-printed robots, in the hope that he will be spared when the revolution inevitably comes.

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