I had issues with this when I was a little girl, like 5 or 6. It was due to high stress in my case, and was diagnosed as Trichotillomania. I had pulled so much hair out that I had to keep a very short haircut for a couple years to keep me from pulling it out. There would be piles of hair by my bed when I'd wake up in the morning. I still have moments where I really want to pull my hair out, but I don't actually do.