The normal hair growth cycle has three distinct phases that take two to three years to complete. The cycle includes a period of active growth, a regression period and, finally, a resting period. At any given time, about 90 percent of your hair is growing at a rate of just under 1/2 inch each month. The remaining 10 percent is in a resting phase that ends when the hair falls out.
Hair Structure
The two structures that are involved in hair growth are the hair follicle and the hair shaft. The follicle, a sac-like structure located below your skin, protects the hair root. The hair is nourished in this protected area by the food and oxygen in your blood and by secretions designed to promote hair growth. The hair shaft itself is made of fibers of keratin proteins and grows out of the follicle.
Growth Period
The growth period of hair, known as the anagen phase, lasts up to several years for scalp hair. The length of the growth phrase is genetically predetermined but is also affected by the size and location of the hair follicle. During this period, the cells in the follicle respond to complex signals transmitted by the skin by becoming very active and dividing at a rapid rate to produce outward growth in the hair shaft.
Regression Period
During the regression period, or catagen phase, the follicle stops producing new protein cells and begins to restructure itself. During this transition, the follicle moves upward, toward the surface of the skin and creates a structure to prepare the hair for its resting phase. The follicle forms a club hair, a non-living thread-like structure that attaches to the hair shaft and caps further growth in that particular hair. This period lasts from two to three weeks.
Resting Period
About 10 to 15 percent of scalp hairs are in a resting period, or telogen phase, at any given time. During the resting period, which lasts for about three to four months, the hair follicle has stopped all activity. The club hair, begun during the regression phase, continues to form and reaches completion. The hair growth cycle ends when the hair is shed, which occurs at a rate of about 25 to 100 hairs each day.
Factors Affecting Growth
Overall good health and a well-balanced diet contribute to healthy hair growth. In particular, sufficient iron, zinc and vitamin B are needed to promote growth. Malnutrition, including the effects of severe dieting and dietary deficits associated with alcoholism, affects hair growth adversely. Hormones also play an important role in hair growth. Male hormones, called androgens, are important for regulating hair growth. Female hormones, or estrogens, slow hair growth but extend the growth period.
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