The Hair Growth Cycle Explained: Anagen, Catagen, Telogen and Exogen

Why the cycle matters

Almost every question about hair loss comes back to the hair cycle. Why does shedding increase when starting Minoxidil? Why does stress-related shedding appear months later? Why do you need to wait six months to judge a product? Why doesn't hair just keep growing forever? Each answer depends on how follicles move through their phases.

The average adult scalp has around 100,000 follicles. Each one is a small organ that repeatedly builds a hair, rests and starts again. Unlike some animals, humans shed asynchronously: neighbouring follicles are at different stages, so we don't moult all at once.

Phase 1: Anagen (growth)

Anagen is the active growth phase. Cells at the base of the follicle divide rapidly, building the hair shaft and pushing it upward. On the scalp, anagen normally lasts from about two to seven years, and at any time around 85-90% of scalp hairs are in this phase.

The length of anagen largely determines how long your hair can grow. Someone with a long anagen phase can grow hair to waist length. Someone with a shorter phase may find hair seems to stop at shoulder length. Eyebrows and body hair have much shorter anagen phases, which is why they stay short.

Phase 2: Catagen (transition)

Catagen is a brief transition lasting around two to three weeks. Growth stops, the lower part of the follicle shrinks and the hair detaches from its blood supply, forming a club hair. Only about 1-2% of scalp hairs are in catagen at any time.

Phase 3: Telogen (rest)

Telogen is the resting phase, lasting about three months. The club hair stays in the follicle while the follicle prepares to begin a new anagen phase. Around 10-15% of scalp hairs are normally in telogen. When larger numbers are pushed into telogen together, as in telogen effluvium, noticeable shedding follows a few months later.

Phase 4: Exogen (shedding)

Exogen is sometimes described separately: the active release of the old club hair, often as a new anagen hair begins to grow beneath it. This is the hair you find in the brush or the drain. Everyday shedding is simply exogen happening across thousands of follicles at different times.

The cycle at a glance

What goes wrong in pattern hair loss

In androgenetic alopecia, susceptible follicles respond to DHT with a progressively shorter anagen phase. Each new hair has less time to grow, so it ends up shorter. At the same time the follicle itself shrinks (miniaturization), producing a finer, lighter hair. The telogen phase can also lengthen, and the gap between shedding and regrowth can widen.

The overall effect, repeated over many cycles, is fewer long thick hairs and more short fine ones, which looks like thinning even before any area becomes bald.

How Minoxidil interacts with the cycle

Topical Minoxidil is understood to shorten telogen, prompting resting follicles back into anagen sooner, and to prolong anagen while increasing follicle size. The first effect explains early shedding: resting hairs are released as new ones start to grow. The second explains why improvements, when they occur, take months to become visible: new hairs have to grow long enough to change how the scalp looks.

Seasonal shedding

Some studies have found seasonal variation in shedding, with many people noticing slightly more hair loss in late summer and autumn. This is a normal variation and is usually mild and temporary. It is worth keeping in mind when comparing photos taken in different seasons.

What this means for your routine

• Allow at least one full cycle of new growth (roughly 4-6 months) before judging any change
• Expect some shedding when starting a routine that affects the cycle
• Take photos in the same season where possible for fair comparisons
• Don't stop and restart repeatedly; each restart resets the clock
• Focus on density and hair thickness over time rather than daily hair counts