Hair grows from a structure called the follicle, which is a narrow tube that travels down from the surface of the skin. At the bottom of the follicle, there is a hair bulb. According to Procter & Gamble, this hair bulb is like a little factory, producing hair growth for decades and in some cases centuries, starting from birth. Hair may be cut, bleached and exposed to sun and wind, but none of these exposures affects the continued growth of hair that comes from the bulb. After a hair stops growing, it falls out, completing its growth cycle.
Three Phases of Hair Growth
The three stages of hair growth have been given names, and each stage has a duration associated with it. The anagen phase is the growth phase of hair. It has a typical duration of three to seven years. The catagen phase of the hair growth cycle is a transitional phase and lasts one to two weeks. The third phase of the hair growth cycle is the resting phase, or the telogen phase, and it can last from three to four months.
Anagen Phase
During the anagen phase of hair growth, the hair bulb at the bottom of the follicle produces new cells. This new growth pushes the shaft of the hair up, causing hair to lengthen. Hair growth occurs at about one centimeter per month. About 85 percent of hair is in the anagen phase at any given time. During the anagen phase, pigment is also made in the hair bulb, which gives the hair color. As people age, less pigment is produced.
Catagen Phase
During the catagen phase, the hair follicle shrinks. No cells are produced during this phase, and so there is no additional hair growth. No pigment is produced during this phase either. The base of the follicle moves upward, toward the surface of the skin, causing the length of the follicle to become shorter.
Telogen Phase
The telogen phase of the hair growth cycle is the resting phase. During this phase, the hair remains attached to the follicle until it sheds. A new hair begins to grow. If the old hair is still in the follicle, it will get pushed out. The new hair grows out from the same opening of the follicle, and the hair growth cycle begins again.
Hormones
Hormones affect the growth cycle of each individual hair. Changes in hormone levels, such as occurs during pregnancy, can affect the hair growth cycle. Levels of the female hormone estrogen surge during pregnancy. Estrogen slows the growth rate of hair but lengthens the period of growth. Hair on the scalp is more sensitive to androgens; known as male hormones, they are present in both males and females. The balance between male and female hormones is the deciding factor of hair growth.
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