Hair growth is cyclical. The hair grows, goes through a transition stage, rests, and then falls out. Sometimes the follicle will enter a latent stage before the whole process begins again.
Anagen (the Growing Phase)
The growing phase has six stages, but the only one you see is the final stage, when the hair emerges from the scalp. Obviously, the longer your hair is in this phase the longer it will grow and the less you will see falling out. Also, your hair length will be determined by the speed of growth, which is, on average, 1.2 centimeters (1/2 inch) per month. The proportion of hair normally in this phase is 80 to 95 percent. Disruptions to the hair cycle during anagen are the main causes of hair loss.
The growing phase is the most relevant time to ascertain hair length. This phase of the hair cycle averages about one thousand days (three years). As the hair grows at an average speed of a half-inch per month (or six inches per year), the average length an individual hair strand grows is approximately eighteen inches. Some people’s growing phases are much longer (more than five years), and some are much shorter (less than two years). This means that some people can grow their hair down to their feet, while others cannot grow it beyond their shoulders.
Catagen (the Transition Phase)
The catagen or transition phase is the point where the hair has stopped growing and the root sheath, which anchors the hair into the follicle, begins to break down. The proportion of hair in this phase is less than 2 percent. This phase is rather insignificant as the hair is either growing (an anagen hair) or isn’t (a resting hair). For this reason, most researchers will count catagen hairs (if they can find them!) in with telogen hairs. This phase lasts, on average, 7-10 days.
Telogen (the Resting Phase)
During the telogen phase, the once-living cells near the bottom of the follicle (called the papilla) become hardened (keratinized) and form a club or bulb-type end. The follicle shrinks away from the papilla and eventually releases the hair (this shedding of the hair is sometimes called the exogen phase). The percentage of hair in the telogen phase is 5 to 18 percent. After about three months, the follicle will move back down to rejoin with the papilla and the whole cycle begins again.
With genetic hair loss, the follicle can go into a latent stage called the kenogen phase before restarting anagen
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