Anagen is the longest phase with up to 90% of follicles on a normal human scalp in this active hair growth state at any given time and correspondingly telogen hair follicles comprise up to 10% on the scalp. The average rate of hair fiber growth is around 0.35mm a day but this rate varies depending on the site of the hair follicle and the age and sex of the individual. The length of the anagen growth phase for scalp hair is usually 3-6 years while telogen lasts just 30-90 days and catagen is best estimated at 14-21 days. In many young mammals the anagen growth phase occurs in a wave like pattern across the skin surface, but the hair follicles of humans can run through the normal cycles of growth apparently independently of neighboring follicles.
Normally this cycle of hair production and inactivity will continue for the duration of the individual's life but other factors can influence and inhibit hair production and in some cases lead to physical destruction of the hair follicle. Factors may include adverse reactions to drugs and cosmetics, or as a result of scarring, tumors, radiation, the genetics of the individual, hormones and/or their immune system.
The first diagram on the left is showing regression of a mature anagen hair follicle. On entering catagen the dermal papilla condenses as the cells become inactive. With a lack of dermal papilla cell stimulation, the hair fiber and root sheaths stop growing. In telogen the dermal papilla can become isolated in the dermis and the hair fiber can easily be pulled out (by combing, shampooing, or brushing)
The secnod diagram on the left is showing a resting hair follicle returning from resting telogen to growing anagen. If the old fiber has not already fallen out it is pushed out by the new hair fiber growing underneath
Article courtesy of Keratin.com