Do hairs know they've been cut?

Question

Hello,

I have a question about body hair. When we remove unwanted body hair by plucking/shaving, we know that they grow back more or less to its natural length.

When we trim them, the hair also seem to grow back to its natural length, but this has mystified me for some time. a) How does the hair know it's been cut without stress caused to the hair follicle, and therefore start growing again; and

b) How does it know when to stop growing when it reaches the intended length?

Many thanks,

Maki Aoyama

Answer

Ginny - So, the short answer is that your hair doesn't know that it's been cut. This is a really common myth. People think that if they trim their head hair when they're trying to grow it, that it'll make it grow faster and people say that if shave your legs or your face, it makes it grow back faster, but that's not actually the case. The only thing that affects how long your hair is and how fast it grows is the hair itself and your genetics. So, hair goes through 3 phases when it's growing. There's the anagen phase which is the growth phase and this is when it actually grows, when it gets longer. Now, on head hair, that can last between 2 and 6 years and how long that phase is, counts as how long your hair could grow to. So, some people seem to be really lucky and they can grow hair down to their waist whereas other people, no matter how hard they try, it will never get pass shoulder length.

Chris - Not their eyelashes though, surely.

Ginny - No, not their eyelashes. Now, that's a point because body hair has a much shorter anagen phase. It's only a couple of months and that's why it never gets as long as head hair. I think there are few exceptions of weird people who've managed to grow their underarm hair ridiculously long in the Guinness Book of Records, but with a couple of weird exceptions, it doesn't get as long as the hair on your head because this anagen phase only last a couple of months. Now after that phase, what happens is the hair is cut off from the blood supply so the follicle sort of dies and it enters a dormant stage and then a bit of time after that, it'll fall out as a new hair begins to grow. So, what you're seeing is you're trimming say, your leg hair while it's still in the anagen phase - it's going to continue growing. But it can only ever get to a certain length because that anagen phase will end and it will fall out.



Source: https://www.thenakedscientists.com/articles/questions/do-hairs-know-theyve-been-cut 

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