Virtually all hair follicles undergo the same life cycle. There are three stages of this life cycle--the anagen phase, the catagen phase and the telogen phase. At any point in time, the hair on a human body is in one of the phases.

Once the cycle is complete, a new strand of hair emerges.

I. The Anagen Stage

In the anagen stage or growth stage the hair follicle produces new cells.

Cell activity is accelerated, working faster than normal cell activity elsewhere in the body. (However, growth rate varies depending on the location on the body, sex of the individual, age of the individual and health of the individual.) Women grow hair more rapidly than men. Scalp hair grows fastest between the ages of 15 and 30, and it slows down after the age of 50. On average, 90 percent of scalp hair is in the anagen stage at any given time. The anagen phase typically lasts three to five year, but can last as long as 10 years. The longer the scalp hair remains in the anagen stage, the longer the hair is able to grow, which is why some people are able to grow their hair extremely long while others are not.

II. The Catagen Stage

The catagen phase is a short, transitional phase that occurs at the end of the anagen growth phase and before the telogen resting phase. During this phase, the follicle renews itself and also shrinks to about 1/6 of its normal size as the papilla detaches. When this occurs the blood circulation is cut off. Less than one percent of scalp hair is in the catagen phase at any one time. This phase is extremely short, lasting just one or two weeks.

III. The Telogen Phase

The telogen phase is also known as the resting phase. In this phase, the hair follicle can remain dormant anywhere from one to four months. About 10 to 15 percent of hair can be in the telogen phase at one time. This is the final phase in the hair cycle, and it lasts until the fully grown hair is shed. The hair either sheds during the telogen phase or remains in place until the onset of the next anagen phase, at which time the new hair pushes the old hair out of the follicle. Once the telogen phase ends, the hair follicle reactivates, returning to the anagen phase, and the entire cycle begins again. This cycle generally repeats itself once every four to five years.