The primary role of hair in animals is to serve as insulation by trapping warm air against the body, and the hair coat in mammals requires a constant supply of new hairs throughout the lifetime of the animal. In humans, hair has lost most of its protective functions and now plays an important role in social interactions. For these very reasons, nature ensures that each hair follicle cyclically regenerates its lower portion that produces the hair shaft.
Although no new hair follicles are made postnatally, each and every hair follicle undergoes three-part cyclical growth pattern in order to produce a new hair. Normally, the hair grows to a maximum length, then hair growth ceases and the hair is shed and replaced. These phases of the hair growth cycle have been described as:
1. Anagen, a long period of growth
2. Catagen, the transitional period from growing to resting lasting 2 to 4 weeks
3. Telogen, a period of inactivity lasting 2-4 months