Hairs are mini organs that experience cyclic activity during adult life. The cycle that each hair undergoes is divided into three phases: the Anagen (growth) phase, the Catagen (regressing) phase, and the Telogen (resting) phase. Individual hairs go through the latter three phases of this cycle independent of other hairs.

Breaking Down Hair’s Three Phases

Anagen Phase:  During the Anagen phase, hairs grow roughly one centimeter every four weeks. This process lasts for two to seven years, based on predisposed genetics. Most of our hairs (roughly 85%) are undergoing the Anagen phase at any given time.

Catagen Phase: In contrast to the long-lasting Anagen phase, the Catagen phase only lasts one to three weeks. During this phase, hairs shrink and are slowly cut off from their blood supply, leading to detachment from the growth cells that fuel the Anagen phase.

Telogen Phase: The Telogen phase is the five-to-16 week resting period where the hair stays attached to the follicle (hair root) but does not grow further. This is the final stage before the hair “sheds” (as you may experience in the shower). Follicles re-enter the Anagen phase following the end of the Telogen phase.

Roughly 25 to 100 hairs are shed each day, and hair loss can be attributed to the exaggeration or interruption of the three phases. For example, added stress can rush hairs from the Catagen phase to the Telogen phase, causing rapid hair loss. Additionally, Telogen Effluvium, the second most common form of hair loss, can be caused by follicles staying in the Telogen phase without being able to re-transition to the Anagen phase.

Dr. Ryan Welter is the founder of New England Center for Hair Restoration and a leader in the field of hair restoration. Follow him on Twitter @DrRyanWelter.