Hair growth cycle
Each hair follicle lives in a cycle consisting of a long period of growth followed by a relatively short period of rest. During the rest period the hair is still attached to the hair follicle, but it is not growing. After the resting phase the hair is shed and a newer hair begins to grow, thus, starting a new hair growth cycle.
There is a genetic predisposition of each hair follicle to follow a particular pattern of growth and rest. Hair follicles are programmed to stop producing hair and spend more time in the resting stage with the progression of age.
The growth cycle of a hair consists of 3 stages:
This is the name for the growing period of a hair follicle. The anagen stage for the hair follicles in the scalp typically lasts about 3 to 5 years.
At the end of the growth period, hair follicles prepare themselves for the resting phase. This transition period of a hair follicle from growth to rest is called the catagen stage. This stage of the hair growth cycle usually lasts about 1 to 2 weeks or so. During the catagen phase the deeper portions of the hair follicles start to collapse.
This is the resting period of a hair follicle. It is usually 3 to 4 months in length and at the end of this period older hairs that have finished their life will fall out and newer hairs will begin to grow.
The growing (anagen) phase constitutes about 90% (1000 days or more) of the growth cycle of a hair follicle, while intermediate (catagen phase; 10 days) and shedding (telogen phase; 100 days) phases constitute only 10% of it. That is, at a given time, about 10% of hair follicles are in the intermediate and shedding phases; thus, not growing. These hair follicles, however, are randomly distributed over the scalp, so that no bald spots are seen.