All hair follicles are replaced at different rates by the normal process of hair cycling. Hair growth alternates between phases of activity and rest. The growth period, called the anagen phase, lasts for two to six years. During this time, the follicle is long and deep and produces thick, well-pigmented hair.
About 90% of all scalp hairs are in the anagen phase at a given time.
Anagen is followed by a brief transition phase known as the catagen phase, which lasts 1–2 weeks. During this time, the base of the follicle shrivels. The resting period, or telogen phase, follows catagen and lasts for three months. In this phase, the shrunken follicle retains the hair fibre.
Following the telogen phase, the next anagen phase begins, and the old hair is dislodged and falls out to make room for new hair to begin growing in its place.