Reviewed by Clare Devereux · IAT Certified Clinical Trichologist · August 2026

The hair growth cycle is the repeating sequence of growth, transition, and rest that every hair follicle moves through independently. It has three phases: anagen (active growth), catagen (transition), and telogen (rest and shedding). The length of the anagen phase — which lasts between two and seven years — determines how long and thick each hair can become. When the cycle is disrupted by stress, nutritional deficiency, hormonal changes, or scalp conditions, the visible result is thinner, shorter, weaker hair or increased shedding.

The three phases of the hair growth cycle

Anagen is the active growth phase. The follicle is fully engaged, the hair fibre is being built from the root, and the strand lengthens at roughly one centimetre per month. At any given time, around 85 to 90 per cent of the hair on your head is in anagen. The longer this phase lasts, the longer and stronger the hair grows. Genetics set the upper limit, but nutrition, scalp health, and hormonal balance determine whether your follicles reach it.

Catagen is a brief transitional phase lasting two to three weeks. The follicle shrinks, the blood supply to the hair root reduces, and growth stops. This is the natural signal that the current cycle is ending.

Telogen is the resting phase, lasting around three months. The hair remains in the follicle but is no longer growing. At the end of telogen, the hair is released — this is the shedding you see on your brush, your pillow, or in the shower. Losing 50 to 100 hairs a day is normal and simply reflects follicles completing their cycle. When the old hair sheds, a new anagen phase begins in the same follicle and the cycle starts again.

Why the hair growth cycle matters

Most changes in hair quality — thinning, increased shedding, loss of density — trace back to something happening within the cycle. Stress can push a large number of follicles into telogen simultaneously, causing a wave of shedding months later. Low ferritin can shorten the anagen phase, producing thinner hair each cycle. Hormonal shifts can trigger follicle miniaturisation — a progressive shortening of anagen that leads to finer, shorter strands over time.

Understanding the cycle is also the foundation of hair longevity: the practice of keeping hair in its healthiest growth phase for as long as possible. A trichologist assesses the cycle directly — using trichoscopy and blood biomarkers — to identify where it has been disrupted and what is driving the change. (For a detailed overview of hair follicle physiology, see this StatPearls hair physiology reference.)

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Frequently asked questions

A complete cycle — from the start of anagen through catagen and telogen to the shedding of the hair — typically takes two to seven years, though this varies between individuals and is influenced by genetics, age, nutrition, and hormonal status. The anagen phase accounts for the vast majority of that time; catagen lasts two to three weeks and telogen around three months.

You cannot meaningfully increase the rate at which hair grows (roughly one centimetre per month), but you can protect and extend the anagen phase — which determines how long and thick each hair becomes. Addressing nutritional deficiencies, maintaining a healthy scalp environment, and managing hormonal or stress-related disruption all help keep follicles in their active growth phase for longer.

Increased shedding usually means a larger-than-normal number of follicles have entered the telogen phase at the same time — a condition called telogen effluvium. Common triggers include significant stress, illness, nutritional deficiency, hormonal changes, or medication. The shedding typically appears two to three months after the trigger event, because that is how long the telogen rest phase lasts before the hair is released.

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IAT-Certified Clinical Trichologist

Clare Devereux

Clare is the founding clinical trichologist at Hair Health Essentials, practising in Dublin and London. With over a decade of clinical trichology experience underpinned by a lifetime in professional hair and scalp care, she specialises in personalised diagnostics — from trichoscopy and blood biomarker analysis to genetic testing — to identify what’s really happening with your hair and scalp.

Clinics: Blackrock, Dublin · Eden One, Dublin · Harley Street, London

Hair Health Essentials provides specialist trichological guidance. This content is for informational purposes and does not constitute medical advice. If you have concerns about your health, please consult your GP or medical practitioner.

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