Hair grows in a cycle that consists of three phases: Anagen, Catagen and Telogen
Anagen - This is the growth phase that lasts between two and eight years. During the anagen phase the growth cells in the papilla rapidly divide and produce the hair shaft which becomes keratinized as it pushes up and out of the follicle into the pore. At the same time, the follicle grows down into the deeper levels of the dermis (skin) to get nourishment. People who have long anagen growth rates are able to grow very long hair and others have short growth phases and cannot grow very long hair. Hair grows at a rate of about a ½ inch per month, so a hair left uncut will grow to a length of between 12 inches and 48 inches.
Catagen - The Anagen phase is followed by a brief two to four week Catagen phase or transitional phase. This is part of a renewal process where the follicle is literally degraded and the hair stops growing but does not fall out. During the Catagen phase the hair follicle shrinks to about 1/6 of the normal length. The lower part is destroyed, the dermal papilla breaks away, the bulb detaches from the blood supply and the hair shaft eventually is pushed up as the follicle disintegrates.
Telogen - The follicle then goes into the Telogen or resting phase for two to four months, during this time the hair still does not grow but remains attached to the follicle while the dermal papilla is in a resting phase below. Approximately 10-15 percent of all hairs are in this phase at any one time.
After the Telogen phase the cycle is complete and the hair goes back into the Anagen phase. It is at this time when the new hair shaft is forming that the old hair is pushed out and lost.
On average 50-100 hairs are lost due to this natural growth process every day. This is normal hair loss and accounts for the hair loss seen every day in the shower and with hair combing. In healthy follicles these hairs will soon be replaced by new hair.
A variety of factors can alter the normal hair growth cycle and cause temporary or permanent hair loss including medication, radiation, chemotherapy, exposure to chemicals, hormonal and nutritional factors, thyroid disease, generalized or local skin disease, and stress.