[thedoc (OP)]

Kerry Smith asked the Naked Scientists:
   
Why does the hair on my head continue to grow and grow Rapunzel style, yet the hair on my legs, chest (Thankfully, I'm a guy) and unmentionables seems to grow to a certain length and then stop.  

What chemical or physical attributes do these follicles have that would allow them to sense how long they are?

What do you think?

[evan_au]

It is due to a genetic cycle, where the hair grows for a certain time, then rests.
- When a new cycle begins, the old hair falls out.
- Different parts of the body have hair follicles which work on a different cycle time, which produces hairs of different lengths on different parts of the body.

I understand that progress has been made in understanding the genetic clocks that control the daily circadian rhythm. Maybe soon they will understand the clocks controlling hair growth - and then you can control the length of your hair without going to the barber?

[snorkfort]

My chest hair doesn't have this problem. It continues to grow.
On the other hand, the hair on my head has permanently ceased to grow in some places.

Another possible answer could be that hair tends to fall out now and then, and the number of hairs that fall out per day tends to keep your hair from growing beyond a certain length, and that length differs depending on the body part. Otherwise cave men and cave women would have been walking around with hair down to their ankles or worse.