How does the hair growth cycle work?

The answer

Hair growth follows a cycle with three main phases. The anagen (growth) phase lasts 2 to 7 years for scalp hair, during which the hair follicle's matrix cells divide rapidly, adding about 1 centimeter of length per month. The catagen (regression) phase lasts 2 to 3 weeks, during which the follicle shrinks to one-sixth of its normal length and the hair detaches from its blood supply. The telogen (resting) phase lasts about 3 months, after which the old hair is pushed out by a new hair entering anagen. At any given time, about 85-90% of scalp hairs are in anagen, 1-2% in catagen, and 10-15% in telogen. It is normal to shed 50 to 100 telogen hairs daily. Different body regions have different cycle lengths, which is why eyebrow hair stays short while scalp hair can grow very long.