What Are the Phases of the Hair Growth Cycle?

Hair growth is not a continuous, linear process, it is a cyclical biological phenomenon governed by distinct phases that repeat throughout life. A comprehensive understanding of the hair growth cycle is essential for clinicians, patients considering hair restoration, and anyone interested in scalp and follicular health. Clinically, disturbances in this cycle underlie many common forms of hair loss, including telogen effluvium and androgenetic alopecia.

Overview of the Hair Growth Cycle

The human hair follicle undergoes a cyclic process consisting of multiple distinct phases: anagen, catagen, telogen, and the emerging conceptual fourth phase, exogen (shedding). Each phase reflects a different biological state of the follicle, with implications for hair length, strength, and turnover.

Anagen (Growth Phase): Anagen is the active growth stage of the hair cycle. During this phase, hair matrix cells rapidly proliferate and the hair shaft elongates.

• Longest phase of the hair cycle, especially in scalp hair (typically 2–7 years).
• Responsible for determining maximum hair length.
• At any given time, approximately 85–90% of scalp hairs are in anagen in healthy individuals.
• Disruption or shortening of anagen (as seen in androgenetic alopecia) leads to shorter, thinner hairs and eventual hair thin

Catagen (Regression Phase): Catagen is a brief transitional phase marked by controlled follicular regression. Apoptosis (programmed cell death) begins to involute the lower follicle.

• Typically lasts 1–3 weeks.
• The hair shaft disconnects from the blood supply as the follicle shrinks.
• Only a small proportion (≈1–2%) of follicles are in catagen at one time under normal conditions.

Telogen (Resting Phase): Telogen represents the resting and preparatory stage of the hair cycle. In this phase, the follicle remains inactive, and the old hair persists but ceases growth.

• Typically lasts about 2–4 months.
• During telogen, follicles are not growing new hair shafts but may still be metabolically active.
• At the end of telogen, shedding of the old hair occurs, making room for new anagen hairs.

Exogen (Shedding Phase): Exogen is sometimes conceptualized as a discrete phase of hair release. It overlaps the end of telogen when old hairs detach and are shed

Conclusion

The hair growth cycle is the biological rhythm underlying visible hair renewal and shedding. Its phases, anagen, catagen, telogen, and exogen, reflect sophisticated regulatory networks that control follicular behavior. Comprehensive knowledge of these phases enhances diagnosis, patient education, and therapeutic outcomes in clinical hair restoration.