Hair Growth Cycle (Anagen–Catagen–Telogen)

Human hair does not grow continuously. Instead, each hair follicle follows a biological cycle consisting of distinct phases that regulate growth, rest, and shedding. This cycle explains why hair length, density, and regrowth timing vary between individuals and across different scalp regions.

Understanding the hair growth cycle is essential for interpreting hair loss patterns, evaluating medical treatments, and setting realistic expectations after hair transplant surgery.

Overview of the hair growth cycle

The hair growth cycle consists of three primary phases: anagen, catagen, and telogen. Each follicle progresses through these phases independently, which is why not all hairs shed or grow at the same time.

A general explanation of surgical hair restoration can be found here:
What Is Hair Transplant?

Anagen phase: active growth

The anagen phase is the active growth stage of the hair cycle. During this phase, cells in the hair bulb divide rapidly, producing a visible hair shaft.

• Duration: typically 2–7 years on the scalp
• Percentage of scalp hairs: approximately 85–90%
• Determines maximum hair length

Genetics strongly influence the length of the anagen phase. A longer anagen phase allows hair to grow longer and thicker.

Catagen phase: transition

Catagen is a short transitional phase marking the end of active growth. Cell division slows, and the hair follicle begins to shrink.

• Duration: about 2–3 weeks
• Percentage of scalp hairs: less than 1%

Although brief, the catagen phase is essential for resetting the follicle and preparing it for rest.

Telogen phase: resting and shedding

During the telogen phase, the follicle enters a resting state. The hair shaft remains in place temporarily before being shed and replaced by a new anagen hair.

• Duration: approximately 2–4 months
• Percentage of scalp hairs: around 10–15%

Daily shedding of 50–100 hairs is considered normal and reflects healthy cycling.

Exogen: the shedding event

Some experts describe exogen as a separate shedding phase. It refers specifically to the release of the hair shaft from the follicle.

Exogen explains why shedding may occur without visible hair loss, as new hairs are already forming beneath the skin.

Why follicles cycle independently

Independent cycling prevents simultaneous shedding of all hair, which would result in visible baldness. This asynchronous pattern maintains overall scalp coverage.

Hair growth cycle and hair loss

Disruptions in the hair growth cycle can lead to hair thinning or shedding disorders. In androgenetic alopecia, the anagen phase shortens progressively, producing finer hairs over time.

Hair growth cycle and hair transplantation

Transplanted follicles retain their biological programming and follow the same growth cycle after surgery. This explains the temporary shedding phase commonly seen after transplantation.

The stages of post-transplant growth are discussed here:
Hair Transplant Growth Stages

Why transplanted hair sheds initially

Surgical stress often pushes follicles into a temporary telogen phase. The hair shaft falls out, but the follicle remains viable and capable of regrowth.

Timeline of regrowth after transplantation

After shedding, follicles gradually re-enter anagen. Visible regrowth usually begins around the third or fourth month, with continued maturation over a year or longer.

Factors influencing the hair growth cycle

• Genetics
• Age
• Hormonal balance
• Scalp health
• Systemic health conditions

Clinical relevance of growth phases

Understanding growth phases helps clinicians distinguish between normal shedding and pathological hair loss. It also guides timing for treatment evaluation.

Patient expectations and education

Educating patients about the hair growth cycle reduces anxiety during shedding phases and improves satisfaction with long-term outcomes.

References

1. DermNet NZ – Hair Growth Cycle
   
2. NIH – Biology of Human Hair Growth
   
3. JAAD – Human Hair Cycle and Hair Loss Disorders
   

Medical Disclaimer:
This article is provided for informational and educational purposes only and does not constitute medical advice.
It is not intended to replace a face-to-face consultation, diagnosis, or treatment by a qualified physician.
Individual treatment decisions should always be made in consultation with a licensed medical professional.