The Hair Growth Cycle Explained

Every hair on your scalp goes through a repeating cycle: a long growing phase (anagen), a short transition (catagen), a resting phase (telogen) and the release of the old hair (exogen). Each follicle runs on its own schedule, which is why you shed a little every day rather than all at once. Most types of hair loss can be understood as a disruption to one or more of these phases.

Why the cycle matters

Hair follicles are among the most active structures in the body. Unlike skin, which renews continuously, a follicle grows a hair for a period, then shuts down, rests and starts again. Understanding this rhythm helps explain several things that otherwise seem puzzling: why shedding often appears months after a stressful event, why hair treatments take so long to show results, and why some hair loss recovers while other types do not.

The four phases

Anagen: the growing phase

Anagen is when the follicle is actively producing hair. Cells at the base of the follicle divide rapidly, pushing the hair upward. On the scalp, anagen typically lasts somewhere between two and seven years, and at any one time most scalp hairs, often around 85 to 90 per cent, are in this phase.

The length of anagen largely determines how long your hair can grow. Scalp hair grows roughly a centimetre a month. Someone with a long anagen phase can grow hair well past the shoulders, while a shorter anagen limits maximum length. Eyebrows and eyelashes have much shorter anagen phases, measured in weeks to months, which is why they stay short.

Catagen: the transition

Catagen is a brief phase, lasting about two to three weeks. Growth stops, the lower part of the follicle shrinks, and the hair detaches from its blood supply, forming what is called a club hair. Only a small fraction of hairs are in catagen at any one time.

Telogen: the resting phase

In telogen, the follicle rests and the club hair stays in place, but does not grow. On the scalp, telogen lasts about three months. Roughly 10 to 15 per cent of scalp hairs are usually in this phase. Towards the end of telogen, a new hair often begins forming beneath the old one.

Exogen: the shedding phase

Exogen is sometimes described as part of telogen and sometimes as a separate phase. It is the active release of the old club hair from the follicle, often helped along by washing or brushing. Shedding 50 to 100 hairs a day is generally considered within the normal range. A shed hair usually has a small, pale bulb at the root.

Some researchers also describe a "kenogen" interval, when a follicle sits empty for a time after shedding before new growth appears. This gap may be longer in pattern hair loss.

A mosaic, not a wave

In some animals, all hairs cycle together, which produces seasonal moulting. Human scalp follicles cycle independently, a pattern often described as a mosaic. That is why a normal scalp looks consistently full: at any moment, most follicles are growing, a few are resting, and a few are shedding. Factors such as daylight may nudge the timing slightly, which may explain the mild seasonal shedding some people notice, covered in our article on seasonal hair shedding.

How disruptions explain hair loss

Shedding conditions: too many follicles resting at once

In telogen effluvium, a shock to the body, such as illness, surgery, childbirth, severe stress or crash dieting, pushes many follicles from anagen into telogen at the same time. Because telogen lasts around three months, the shedding appears well after the trigger. The follicles usually remain healthy and return to growth.

Pattern hair loss: a shortening growth phase

In androgenetic alopecia, genetically sensitive follicles respond to the hormone DHT. Over many cycles, anagen gets shorter and the follicles become smaller, producing finer, shorter hairs. This process is called miniaturisation. Medicines such as minoxidil and finasteride appear to work partly by prolonging anagen or slowing miniaturisation, which is one reason they take months to show any change. Our article on follicle miniaturisation explores this in more depth.

Sudden loss of growing hairs

Some events affect follicles during anagen itself. Many chemotherapy medicines target rapidly dividing cells, which can cause growing hairs to break or fall out within weeks; this is called anagen effluvium. Alopecia areata, an autoimmune condition, also interrupts the growth phase, typically causing round patches. In both cases, follicles often survive and can regrow, although this varies.

Scarring conditions: the cycle ends

In scarring alopecias, inflammation damages the stem cells that allow a follicle to restart its cycle. Once that happens, the follicle is replaced by scar tissue and cannot regrow hair. This is why early assessment by a physician, and often a dermatologist, is important for scalp redness, pain or shiny patches.

What this means for treatment

Because hair cycles slowly, any change, whether natural recovery or a response to treatment, takes time. Most hair loss treatments need at least six months, and often a year, before their effect can be fairly judged. Some treatments may cause a temporary increase in shedding early on, as resting hairs are released to make way for new growth.

The cycle also explains why a correct diagnosis comes first. Shedding caused by low iron needs a different response from pattern thinning, and neither should be treated the same way as a scarring condition. If you are noticing a change, your family doctor is a good first stop, particularly if you have other symptoms.

Frequently asked questions

Can I make the growing phase longer?

Some medical treatments for pattern hair loss appear to extend anagen, but their effect is modest, varies from person to person and lasts only while the treatment is used. General health, including adequate nutrition, also supports normal cycling.

Does cutting hair make it grow faster?

No. Growth happens in the follicle beneath the skin. Trimming removes split ends, which can reduce breakage, but it does not change the cycle.

Why do some hairs grow back thinner?

In pattern hair loss, each new cycle may produce a smaller hair as the follicle miniaturises. In early regrowth after shedding, new hairs are also fine at first and thicken over time.

If you would like to understand which part of the hair cycle may be affected in your case, HealthPark Medical Aesthetics can assess your scalp, explain the likely cause in plain terms and discuss whether treatment, monitoring or a referral makes the most sense.