The life cycle of the hair or that of the hair follicle is divided into two periods. They are the period of growth and the period of rest. The former period is comparatively longer than the later period. Once the period of growth is over the follicle enters into the period of rest. During this period hair completely stops growing and finally falls down, giving way to a new hair that has its own growth cycle.
In general, the hair follicles are programmed to spend a longer rest period as people age. However, people are differentially programmed as per their genes and so are their hair follicles programmed to follow different pattern of period of growth followed by the period of rest and the time span dedicated to the respective periods.
The life cycle of a hair strand is further divided into the following three stages: Anagen, Catagen and Telogen.
This stage is related to the growing period of the hair follicle. As a rough estimate, a hair follicle buried in the scalp spends 3 to 5 years of Anagen stage.
Catagen stage off the life cycle of the hair follicle is witnessed by collapsing of the interior part of the follicle. During this stage, follicle prepares itself for the period of rest. This is basically a stage of transition from growing period to the period of rest and usually lasts for not more than 2 weeks.
Telogen or the resting stage completes one life cycle of a strand of hair. It lasts for not more than 4 months. At the end of the resting stage, which is also the stage of shedding, hairs that have completed their life cycle will fall off and will pave way for growth of new strands of hairs.
At any given time almost 10 percent of the hair follicles are in the stage of Telogen. That implies, one tenth of the total hair is either going to fall or has already fallen. However, the pattern of the shedding is not focused or concentrated and hence it the stage of Telogen does not result in periodic baldness or patch on the scalp.