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This question already has an answer here:

I'm still not quite sure if head hair grows endlessly, or if it stops at some point, but that's not the question.

When the hair on my leg grows, it obviously doesn't grow endlessly, but it also stops at some point. I learnt that hair is dead matter, so why does the body know that my hair reached maximum length, and realizes I cut it when I did?

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marked as duplicate by rg255, kmm, AliceD, WYSIWYG Oct 31 '16 at 7:05

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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That's an awesome question! Let me help you rephrase, as I don't have a direct answer; why does hair stop growing at certain lengths, and differentially at various locations on the body?

There are three stages to hair growth:

  • Anagen phase- growth
  • Catagen phase- transition
  • Telogen phase- rest
  • (then falls out!)

You're asking why and how the anagen phase stops, or more concisely, what processes cause anagen phase to cease and proceed to transition to a degenerated rest.

The indirect answer I have for you is this: there doesn't exist a definitive mechanism for how the the telogen phase is induced- that is to say that the process by which the hair-producing follicle "turns off" it's cycle to come to a halt has not yet been fully elucidated in the literature and remains a discovery to be made.

I hope I've helped you understand your targeted question

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