All the hairs on our body go through a three-phase cycle. In the actively-growing phase, cells deep within the hair follicle multiply quickly and die. As more cells are produced, the dead cells get pushed out of the follicle. These dead cells form a hair. After a brief intermediate phase, the growth stops and the hair falls out. Then the cycle begins again, and another hair starts growing.
It’s the length of the active-growing phase that determines the length of the hair. Hair follicles in the scalp have a growth phase that lasts about three to six years. That’s why some people have hair long enough to sit on. Eyebrow hair, on the other hand, only grows for a few months before it stops and falls out.
Scientists don’t know all the factors that influence the length of the hair growth phase. Sex hormones help regulate the growth rate of hair on certain parts of the body. Sex hormones are also partly responsible for male pattern baldness – in which hair grows for shorter and shorter periods.
If you see a bit of hair in your shower drain, you probably shouldn’t worry. Most people have about a hundred thousand hairs on their head alone – and lose fifty to a hundred hairs a day.