How Does Hair Know When to Stop Growing?

Have you ever wondered how a hair knows when to start and stop growing?

You know what I mean: why do eyelashes grow to a certain length? If you cut one of those small hairs on your forearm in half, somehow it knows it's been cut in half and grows to full length again--but only to it's normal length, and then no more. And why do some hairs grow really long (like those on your head) and some barely at all (like those on the inside of your wrist)?

I used to wonder this all the time.

One day, I decided to find out.

So, your hair grows out of what's called a hair follicle. This is basically a piece of skin that squishes old cells together and makes hair (what scientists call "keratin filaments"). With the follicle, there's this thing called the sebaceous gland. It produces an oily-waxy matter called sebum.

(Side note: The more hair, the more sebum. And the most sebaceous glands are found on your face and scalp, which is why your hair on your head gets greasy and your face gets oily.)

These glands are all over your body except your lips, palms, and the bottoms of your feet.

That's all great, right? But that still doesn't explain why the hair stops growing.

Well, these little hair-making machines, these follicles, like other machines can't run full-time without running themselves into the ground. They go through three phases: anagen (hair is growing), catagen (hair stops growing and falls out), thelogen (follicle recovery time before beginning production again).

So, here's the answer, are you ready???

The three phases run different lengths depending where they are on the body.

And the length of the phases dictates the length of time the hair has to grow.

So, an eyelash has an anagen phase of about a week or so, a pubic hair a few weeks, and a head-hair several years.

The varying lengths are mostly due to the genetic hard-wired info in the follicle, but sometimes hormonal or metabolic control (basically, changes in your body's chemicals).

That's why stress, malnutrition, and puberty can all affect the growth of hair on your body.

Now you know.

Just don't cut yourself shaving--that's a whole different matter.

Please comment below if you thought this was helpful!

Lastly...

there seems to be a pattern over there ------->

(and up...yeah, you have to scroll a bit)

Also seen right here:

To maintain this pattern, I either need to stop posting for the remainder of this year, post at least 117 times, or reach 171 posts (doubt I'll get to 711)...

I'm just saying.

Sources:

The Naked Science Forum

Wikipedia

second image, original source unknown