The hair growth cycle: anagen, catagen and telogen

Pull a hair out and look at the end that was under your skin. If there is a small white bulb, that hair was in its resting phase and was on its way out anyway. If the end is thin and tapered with no bulb, it did not come from the root at all — it broke.

That single distinction resolves a surprising number of hair panics, and it exists because of the cycle every follicle on your head is running independently right now.

The three phases of the hair growth cycle

Anagen — growing. The follicle is actively producing fibre, at roughly a centimetre a month. This phase lasts between two and seven years on the scalp, and its length is mostly genetic. It is also the single most important number about your hair, because it sets your terminal length: at a centimetre a month, a three-year anagen gives you about 36 cm of hair and a six-year anagen gives you 72 cm. People who "cannot grow their hair past their shoulders" usually have a short anagen phase, and no product changes it.

Catagen — shutting down. A transitional phase of two to three weeks. The follicle stops producing, the base regresses, the hair detaches from its blood supply and becomes a club hair: keratinised, finished, held in place mechanically.

Telogen — resting. About three months. The club hair sits in the follicle doing nothing while, underneath it, the follicle prepares to start again. At the end, the new anagen hair pushes up and the old one is released. That release — technically exogen — is what you find in your brush.

Crucially, every follicle runs this cycle on its own schedule. In humans the cycles are desynchronised, which is why we shed continuously rather than moulting. Roughly 85 to 90 per cent of scalp hairs are in anagen at any time and around 10 per cent in telogen; with about 100,000 hairs on a scalp, that arithmetic lands neatly on the familiar 50 to 100 hairs a day.

What the cycle explains

Why shedding spikes have a three-month delay. A stressor — illness, surgery, a crash diet, childbirth, high fever, severe emotional stress — can push a large group of follicles into telogen simultaneously. Telogen lasts about three months. So the shedding arrives roughly three months after the event, by which time you have usually recovered and forgotten it, and it feels like it came from nowhere. That mismatch is the entire reason stress shedding is so frightening and so frequently misattributed to whatever product you started last week.

Why treatments take months to judge. Anything that acts on the follicle can only act on the follicles currently able to respond, and its output has to grow far enough to be visible. Six months is the standard evaluation window in hair trials, and it is not conservatism — it is the cycle. A product judged at four weeks has been judged on nothing.

Why shedding gets worse before it gets better. Treatments that push follicles from telogen into a new anagen phase do so by releasing the old club hairs first. That is the "dread shed" people report on minoxidil, and it is a sign of the mechanism working, which is small consolation while it is happening.

Why seasons matter. Studies tracking healthy people through the year have consistently found a summer peak in the proportion of telogen hairs, with shedding following about three months later — which is why so many people report an autumn shed. It is a real pattern with a real signature, and it is the most common cause of the "help, I am suddenly losing hair" search in September.

Three follicle-level things people confuse with the cycle

Breakage is not a cycle event at all. The follicle is still growing the hair; the fibre is failing somewhere along its length. Same appearance in the sink, entirely different fix. Check the bulb test before doing anything else.

Miniaturisation is a cycle distortion: under the influence of DHT in genetically susceptible follicles, each successive anagen phase gets shorter and the hair produced gets finer and shorter, until the follicle produces only vellus hair. This is androgenetic hair loss, it is progressive, and it is the one where medical treatment early is worth far more than any routine.

Scarring alopecias destroy the follicle outright. There is no cycle to restart, which is why they are the emergency in this field.

What to do with this

Three practical consequences.

Stop evaluating anything at four weeks. Pick one change, hold everything else still, and judge it at three months with photographs taken in the same light — which is the only reason progress photos are worth the trouble.

When shedding increases, look backwards, not sideways. The question is not "what did I start recently" but "what happened to me around three months ago". That reframing turns an unsolvable mystery into an answerable question about half the time.

And separate the length question from the volume question. If your hair will not pass your shoulders, either your anagen phase is short — in which case nothing will change it and you should stop buying things — or your ends are breaking, in which case the damage guides are where the actual answer is.