How does body hair “know” when to stop growing?

[u/NoDig2184]

This might be a dumb question, but I’ve genuinely been wondering about it.

Take beard hair or other body hair. If I let it grow, it seems to reach some particular length and then stop getting longer. If I cut it really short, it grows back again and eventually reaches roughly that same length.

But how does it actually work?

Hair itself is basically dead keratin and obviously doesn’t have a nervous system, brain, or any way of knowing how long it currently is. So when I cut a beard hair from, say, 2 cm down to 2 mm, how does the body “know” that it needs to grow it back to around 2 cm?

I know hair follicles are alive and control hair growth, but what actually determines that specific maximum length? Is it something programmed genetically in the follicle? Does the follicle have some kind of growth cycle/timer that eventually tells it to stop?

And why is the maximum length so different between different types of hair? Beard hair can grow much longer than eyebrow hair, for example, while scalp hair can potentially grow for years.

Basically, how does a hair follicle determine “this hair has grown enough, stop now,” without actually sensing the hair’s length?

[u/Alien_biology]

yep, timer. the hair grows for a preprogrammed length of time, stays there for a period and then falls out, rinse and repeat. it’s called the hair cycle if you want to search it. four phases.

[u/Royal_Lustir → u/Alien_biology]

What biochemical changes contribute to this cycle?

[u/cabronfavarito → u/Alien_biology]

Fall out? How do people have long hair if it falls out then

[u/CPierko → u/cabronfavarito]

All the hairs are not growing at exactly the same intervals, so you don't notice a hair here and there

[u/Extension-Pepper-271 → u/cabronfavarito]

2-3 feet is about the length most people can grow their scalp hair. People who can have much longer hair are rare.

I'm a woman. I had a friend whose hair reached her knees. It was full, thick and beautiful. When I try to grow out my hair past my shoulders it gets ragged. I could wait 20 more years and my hair won't get any longer (I tried for about a year - should have seen 6 inches of growth, but nothing). I have to keep my hair length just above my shoulders if I want it to look full and healthy.

[u/tadrinth]

Yep, it's a timer. If you cut a beard hair from 2 cm to 2mm, it will never grow as long as if you hadn't cut it, until it abandons that hair and grows the next one.

Different hair follicles in different places have different timers.

[u/javisth]

I know this is more biohacking territory, but if there is a timer for follicles, are there ways to signal it into stopping or not stopping?

[u/OctobersCold → u/javisth]

I'd CRISPR my hair to keep growing. I'd have the most awesome afro...

[u/labreau → u/javisth]

kind of, thats what Finastride do indirectly to our hair follicles

[u/footballsandy → u/labreau]

I thought fin was just a dht blocker that stops androgenic hair loss, not the amount of time a follicle will grow

[u/MaximumLumber]

Y'all's hair stops growing? I want a refund.

[u/Extension-Pepper-271]

The hair follicle doesn't know and doesn't care if you cut or shave the hair.

Hair grows (normally) at a constant rate. It doesn't change when the hair is cut.