The term hair growth cycle is used to describe the natural pattern of growth and rest experienced with hair. Within the hair growth cycle are three different phases. Anagen, otherwise known as the growth phase, is when the hair is in a growing cycle. The next phase is called catagen; this is the degradation phase, when the hair follicle is degraded and nonproductive. Telogen is the third phase, during which the hair follicle rests and fails to grow or degrade. Over the course of a lifetime, this hair growth cycle will repeat itself many times.
During the anagen phase, the hair follicle produces hair by binding keratin proteins to make individual hairs. As the hair follicle continues to produce more hair, the hair consistently gets pushed out away from the follicle, making the hair longer. This growth phase of a hair follicle typically lasts anywhere from two to eight years. Immediately following the growth phase is the catagen phase.
When the hair follicle is in the catagen phase, it begins to degrade. This phase also serves as a transition period, when the hair follicle prepares itself for the telogen phase. The degradation phase is the shortest of all three phases and typically lasts between two to four weeks. During this stage, the growth cycle completely stops, and a type of hair called club hair is formed. It's only when the club hair is completely formed and detached from the follicle that the third phase, known as the telogen phase, will start.
The telogen phase is the part of the cycle when the hair follicle rests. The hairs in this phase are detached completely from the hair follicle. Normally, these are the hairs that people lose on a day to day basis when showering or combing their hair. On average, a person loses approximately 50 to 100 of these hairs every day. This phase typically lasts two to four months. Toward the end of this resting phase, the hair follicle forms a new hair and begins the anagen phase of the hair growth cycle all over again.
In an average adult, approximately 90 percent of hairs are in the anagen phase, 1 to 2 percent are in the catagen phase, and 10 to 15 percent are in the telogen phase at any given time. When this cycle is interrupted, it can result in either temporary or permanent hair loss. Extreme hair loss can be caused by a number of different things, such as chemical exposures, skin diseases, and genetic predisposition to baldness. When the hair growth cycle is interrupted by a temporary treatment such as chemotherapy, the hair will usually grow back and eventually go back to a normal hair growth cycle. If the cycle is affected by a more permanent condition, like skin disease, hair loss may be permanent.