The Hair Growth Cycle: Understanding Anagen, Catagen, and Telogen

Every strand of hair on your head follows a precise biological cycle. Understanding this cycle explains why you shed 50-100 hairs per day (which is normal), why stress can cause dramatic hair loss months after the event, and how treatments like finasteride and minoxidil actually work.

The Three Phases of Hair Growth

Each hair follicle independently cycles through three distinct phases. At any given time, approximately 85-90% of your hair is in the growth phase, while the rest is transitioning or resting.

Anagen (Growth Phase)

Duration: 2-7 years (average 3-5 years)

Anagen is the active growth phase where the hair follicle is fully functional and producing new hair. During this phase:

• The hair matrix cells are rapidly dividing
• The hair shaft grows approximately 1 cm per month (about 6 inches per year)
• The dermal papilla is at its largest, providing maximum growth signals
• The follicle is deeply embedded in the dermis

The length of your anagen phase is genetically determined and is the primary factor in how long your hair can grow. People with shorter anagen phases have hair that reaches a shorter maximum length, regardless of how carefully they treat it.

In androgenetic alopecia (AGA): The anagen phase progressively shortens with each cycle. Hair that once grew for 5-7 years may eventually only grow for a few months — too short to reach the surface of the skin.

Catagen (Transition Phase)

Duration: 2-3 weeks

Catagen is a brief transitional phase where the follicle begins to shrink and detach from its blood supply:

• Cell division stops
• The hair follicle shrinks to about one-sixth of its normal length
• The dermal papilla detaches and moves upward
• The hair stops growing and is sometimes called a "club hair"

This phase represents about 1-2% of hair at any given time. It's the body's natural way of recycling old follicles and preparing for new growth.

Telogen (Resting Phase)

Duration: 2-4 months

Telogen is the resting phase where the follicle is dormant:

• The club hair is fully formed but not growing
• The follicle rests in place for 2-4 months
• At the end of telogen, the old hair falls out
• A new anagen hair begins to form beneath it

Normally, about 10-15% of your hair is in telogen at any time. This is why you shed hair daily — it's the natural end of the cycle, not a sign of a problem.

Shedding 50-100 hairs per day is completely normal. You're simply seeing telogen hairs completing their cycle.

The Exogen Phase

Some researchers recognize a fourth phase — exogen — which is the active shedding of the telogen hair. This is the phase where you actually see hair falling out, whether in the shower, on your brush, or on your pillow.

Exogen overlaps with late telogen and can last several weeks. It's triggered when a new anagen hair begins to push the old telogen hair out.

What Happens When the Cycle Is Disrupted

Androgenetic Alopecia (Pattern Hair Loss)

In AGA, DHT binding to androgen receptors progressively shortens the anagen phase. The result:

• Decreased anagen-to-telogen ratio: Normal is approximately 12:1. In AGA, this drops to 5:1 or lower
• Progressive miniaturization: Each cycle produces thinner, shorter hair
• Terminal-to-vellus transformation: Thick, pigmented hairs are replaced by fine, colorless ones
• Irreversible loss: Once the arrector pili muscle detaches from the follicle and the follicle fully atrophies, regrowth is no longer possible

Telogen Effluvium (Stress-Related Shedding)

Telogen effluvium occurs when a significant physiological or emotional stressor pushes a large number of anagen hairs prematurely into telogen. The timeline:

1. Trigger event occurs (surgery, illness, childbirth, extreme stress, rapid weight loss)
2. 2-3 months later: Excessive shedding begins as hundreds of hairs enter telogen simultaneously
3. 3-6 months: Shedding may continue as the wave of prematurely shifted hairs completes telogen
4. 6-12 months: Hair growth typically normalizes once the trigger is resolved

The delay is key: Because telogen lasts 2-4 months, people often don't connect their hair loss to the stressor that caused it. By the time they notice shedding, the trigger may have been months ago.

Anagen Effluvium

Anagen effluvium is a more sudden form of hair loss where the growth phase is abruptly interrupted:

• Caused by chemotherapy, radiation, severe toxins, or autoimmune conditions
• Hair falls out rapidly during active growth
• Often affects 80-90% of scalp hair
• Usually reversible once the cause is removed

Factors That Affect the Hair Growth Cycle

Hormones

• DHT: Shortens anagen phase (the primary mechanism of AGA)
• Thyroid hormones: Both hypo- and hyperthyroidism can disrupt the cycle
• Estrogen: Has a protective effect; its decline during menopause can unmask genetic hair loss
• Cortisol: Chronic stress elevates cortisol, which can push follicles into telogen
• Insulin: Hyperinsulinemia increases 5-alpha reductase activity

Nutrition

Hair follicles are among the most metabolically active tissues in the body. They require:

• Iron (ferritin levels above 40 ng/mL are optimal for hair health)
• Vitamin D — Hair follicles contain vitamin D receptors; deficiency disrupts the cycle
• Zinc — Essential for protein synthesis in the hair follicle
• B vitamins — B-6 and B-12 deficiency can cause thinning
• Protein — Severe restriction triggers telogen effluvium within months

Stress

Chronic stress affects the hair cycle through multiple pathways:

• Elevated cortisol directly disrupts follicle cycling
• Increased DHT production from stress hormones
• Telogen effluvium overlay — stress-induced shedding compounds genetic hair loss
• Compromised nutrient absorption from gut inflammation

Medications

Certain medications can disrupt the hair cycle:

• Beta-blockers (blood pressure)
• SSRIs (antidepressants)
• Isotretinoin (acne treatment)
• Anticoagulants
• Anticonvulsants

How Treatments Work With the Hair Cycle

Understanding the cycle explains why hair loss treatments take time to show results:

Finasteride works by blocking DHT production, allowing the anagen phase to gradually lengthen. Because each cycle takes months, visible improvement requires 6-12 months of consistent use.

Minoxidil stimulates blood flow to follicles and prolongs the anagen phase. The initial "dread shed" that many users experience is actually a positive sign — it means telogen hairs are being pushed out to make room for new anagen growth.

PRP therapy delivers growth factors directly to the dermal papilla, supporting the transition from telogen back to anagen.

Low-level laser therapy stimulates cellular metabolism in the follicle, supporting healthy cycling.

When to See a Dermatologist

Consult a dermatologist if you notice:

• Shedding significantly more than 100 hairs per day for more than 2 months
• Sudden, patchy hair loss
• Receding hairline or thinning at the crown (early AGA signs)
• Scalp irritation, redness, or scaling
• Hair loss that doesn't resolve after 6 months

Early intervention is critical — the sooner you address disruption to the hair cycle, the more follicles you can preserve.

This article is for educational purposes only and does not constitute medical advice.