What Happens During the Anagen Phase of Hair Growth

There are three phases of hair growth. The first is the anagen (pronounced: ANN-uh-jin) phase, the second is known as the catagen (pronounced: KAT-uh-jin) phase, and the third stage is called the telogen phase. Some experts refer to exogen, release of the hair shaft, as a fourth phase.

The anagen phase is the active growth phase of hair follicles. During this phase, a hair grows about one centimeter, or about half of an inch, every 28 days.

Also during this phase, the cells in the root of the hair divide rapidly, adding to the hair shaft. Scalp hair stays in this active phase of growth for two to six years. At any time, about 80 percent to 90 percent of the hairs on your head are in the anagen phase.

The amount of time that a hair follicle stays in the anagen phase is genetically determined. Some people naturally have longer anagen phases and can grow their hair very long, while others will never see their hair get much longer than a foot and a half. At the end of the anagen phase, an unknown signal causes the follicle to go into the catagen phase.

The catagen phase is a short transition stage that occurs at the end of the anagen phase. It signals the end of the active growth of a hair. The hair detaches from its blood supply during the catagen phase. This phase lasts for about two to three weeks while a club hair is formed. 

After the short catagen phase, the hair is released and the hair follicle rests for three months. The club hair falls out. Typically, you lose 50 to 100 hairs per day. After three months, the follicle goes back into the anagen phase and begins to grow a new hair.

It's important to note that all hairs do not go through these stages at the same time. The reason that you don't temporarily go bald is that, at any given moment, some hairs are in the anagen phase, some hairs are in the catagen phase, and some hairs are in the telogen phase. 

People who are on a calorie-restricted diet may shorten their anagen phase. This can also happen due to stress, childbirth, or traumatic events. More hair follicles go into the telogen phase at the same time and you can see diffuse hair loss, known as telogen effluvium. There can also be anagen effluvium from chemotherapy, radiation, or toxic chemicals. These disrupt the hair while it is in the anagen phase. In these cases, the hair will usually recover to its prior fullness. Repeated bouts of dieting or chemotherapy would continue the pattern.

There are rare cases of people who have short anagen syndrome, where they can never grow longer hair for reasons that are unknown. These people will say that they have never needed a haircut.

Loose anagen syndrome is seen in some children. They have sparse hair and their hair is easily pulled out, with the roots showing that they are in the anagen phase. It may be an inherited condition and it usually improves as the child ages.

Some hair products claim to be anagen stimulators that either induce hairs to go into the anagen phase or help hairs stay in the anagen phase longer. Before using any of these over-the-counter products, first talk to a doctor, ideally a dermatologist (a physician who specializes in treating hair, skin, and nails), and ask if there is any peer-reviewed research that supports the claim.