What is Hair?

What is Hair?

Hair growth is a biological process in which hair follicles produce new hair fibers. It’s not continuous; instead, it occurs in cycles, with each hair follicle independently cycling through growth and rest phases. Understanding these phases helps explain why hair length, density, and shedding vary over time. There are three main phases of hair growth: anagen, catagen, and telogen, with a fourth phase sometimes described called exogen.

Hair growth occurs in a repeating cycle of distinct phases. The anagen phase is the active growth phase, lasting years, during which cells in the hair follicle rapidly divide to produce the hair shaft. This is followed by the catagen phase, a short transitional period of a few weeks where growth slows and the follicle shrinks. Next is the telogen phase, a resting stage of around three months, where the hair does not grow but remains in the follicle. Finally, during the exogen phase, the old hair sheds naturally, making way for a new hair to begin growing in the next anagen phase. Together, these phases regulate hair length, density, and shedding in a continuous cycle.

At any given time, around 10-20% of hairs are in the anagen phase while 80-90% at in the catagen or telogen phase. This is typically why 5-8 laser sessions are required, with around 4-8 weeks apart. These downtimes help catch all hairs throughout the anagen phase.

Science Behind Hair Removal

Hair removal—especially laser hair removal—is most effective during the anagen (growth) phase because this is when the hair is actively growing and still connected to the hair follicle. In the anagen phase, the hair bulb contains a high concentration of melanin, the pigment that absorbs laser energy. When the laser targets the melanin, the energy is converted into heat, which damages the follicle and inhibits future hair growth.

In contrast, hairs in the catagen or telogen phases are not actively growing and are either detaching from the follicle or resting. Since they are less pigmented and not connected to the follicle’s growth center, laser energy cannot effectively reach or destroy the follicle, making treatment less effective.