Hair follows cycles of growth then loss in order to grow again. Every pilary cycle renews the hair in 3 stages and can last over a period of up to 5 years. 20 to 25 successive cycles can be observed in a lifetime. For human beings, cycles are not synchronous, thus avoiding the moulting that can be observed in animals with fur.
The 3 pilary cycle stages are:
- The anagen stage during which the hair bulb grows.
- The catagen stage during which the hair bulb is dormant.
- The telogen stage during which the dead hair is expelled by the new hair bulb.