Human hair growth
The three stages of hair growth are the anagen, catagen, and telogen phases. Each strand of hair on the human body is at its own stage of development. Once the cycle is complete, it restarts and a new strand of hair begins to form. The rate or speed of hair growth is about 1.25 centimetres or 0.5 inches per month, or about 15 centimetres or 6 inches per year.
Hypertrichosis
Another mechanism involves a change in the hair cycle. There are three stages of the hair cycle: the anagen phase (hair growth), the catagen phase (hair follicle death), and the telogen phase (hair shedding). If the anagen phase increases beyond what is normal, that region of the body will experience excessive hair growth.