It doesn't know. Hair cycles are constant. If you didn't cut it, it would eventually fall out as it is replaced by another hair. It grows to the point where the epithelial column contracts and starts forcing it out. This is based on environment, nutrition and genetics and varies from person to person and is affected by everything from stress to blood flow.
You can observe this on many people by looking at hair miniaturization of people who have MBP or similar -- the follicle constricts in size and the resulting hair that comes out is progressively shorter in length and diameter until the follicle is so constricted that it no longer produces hair.
It doesn't. Hair grows for a certain amount of time - time, not length. Then waits for a while unchanging, then falls out.
growth (3 to 6 years) -> static (2 to 3 months) -> fall out -> rest -> start over
The maximum length of your hair is determined by how long it stays in the growing phase, in general this phase is longer in females than in males. And some people are naturally able to grow longer hair than others.
In pregnant females the static time is increased, i.e. the hair stays there without falling out. This makes their thick and full. After delivery this changes and the hair falls out in clumps, which is unpleasant, but it's the same hair that would have anyway fallen out earlier, it's not extra hair falling out.
If you cut the hair nothing changes, it still grows for as long as it was going to originally.
That myth about cutting hair (or shaving) to make it grow faster or whatever is not real.
Interesting. I've occasionally wondered why my hair (from scalp) can reach barely half way down my back, even after leaving it for decades uncut, while other people (typically women?) can grow it to the floor.
I've always wondered if the almost universal preference for women to have long hair is linked with their ability to have long hair, while for males people (not you :) typically want it short, which matches what males are able to naturally do.
i.e. which came first? The ability for hair of a certain length, so that's what people like, or if people like hair a certain way so natural selection helps out.
This doesn't seem right. If I use clippers to trim hair on my body, it grows back to its 'natural' length. I don't end up with hair of various lengths. Likewise, if I shave it, I don't end up with some stubble and some regular hairs.
I've never experienced a time where I've trimmed body hair and had it remain the same length for a couple of months.
Look closer, the hairs are random length. You have to wait of course for them to reach natural max length, and then you'll see a randomized mix of lengths.
If you are always clipping hair they will almost all be at one length because > 90% of them are still growing and then getting trimmed. The rest (a minority) have recently fallen out and are just getting started and will be shorter.
> This doesn't seem right. If I use clippers to trim hair on my body, it grows back to its 'natural' length. I don't end up with hair of various lengths.
Are you sure? I have fairly hairy body so when I use moisturizers it tends to stick to the hair instead of the skin, and since I'm trying to take care of my forearm tattoos, I trim my forearm hair on a regular basis, after a few weeks, if I look closely, I can see some hairs being around, say, 2cm long, whereas others are just barely growing out.
IIUC, it doesn't. Hair grows for a certain time period, falls out, and regrows. The follicle has no knowledge of the length of the hair it is producing, it is simply on a grow-stop-shed cycle with a certain period.
There are possible physical answers; it could be lack of inertia on the hair triggers growth. It could be that when you rub your arm, an action as innocent as a yawn, you are also informing the growth of hair. Could it be as simple as looking at your hair or knowledge of shaving causes a subconscious trigger? We already know that these high level shortcuts into low level processes exist (Pavlov's dog anyone?)
