Each hair on the human head goes through three separate cycles throughout the course of its existence. The first, called anagen, is the growth cycle, which is followed by catagen and telogen, which are, respectively, the transition phase and the resting phase. Following are further explanations for each one of these phases of the hair growth cycle.
Anagen refers to the phase in which hair is being actively produced by stem cells in the hair follicle. The longest phase in the cycle, anagen usually lasts from between two to seven years and is subject to genetics. Most hair grows at a rate of approximately one centimeter per 28-day cycle during this phase. At the end of this phase, a genetic signal causes the hair to stop growing and to enter into the cessation, or catagen phase.
The shortest phase of the cycle, catagen is a transitional time when hair stops growing. Catagen lasts from between two to three weeks, during which time the hair converts to what is known as “club hair.” Club hair is formed when the upper portion of the hair follicle attaches itself to the hair shaft and becomes cut off from the stem cells that are necessary to produce new growth as well as from the blood supply. Club hair will not grow anymore. After the process of converting into club hair is complete, the hair then enters the final phase of the cycle, telogen.
The resting phase of the hair cycle, this is the time when hair begins to naturally fall from the scalp. Most people lose from between 50 to 100 individual hairs during the course of a normal day. Although telogen is a natural process that most often occurs after the two preceding stages listed above, it can sometimes happen as the result of the body experiencing extreme physiological stress.