What is the hair growth cycle? The anagen, catagen and telogen phases

Why doesn’t hair just keep growing?

A hair is produced in the follicle (hair root) inside the skin; we describe the structure of the follicle in our article on the hair follicle. The follicle does not produce hair nonstop: it works for a while, then shrinks, rests, lets go of the old hair and starts working again. This repeating process is called the hair growth cycle.

In humans, each follicle follows this cycle on its own schedule, so two follicles side by side can be in different phases. That is why, even though some hair falls out every day, the overall density of your hair does not noticeably decrease under normal circumstances.

The cycle also determines how long hair can grow. Because the growth phase on the scalp lasts for years, scalp hair can grow very long; on the eyebrows and eyelashes, this phase is much shorter, so these hairs naturally stay short.

What are the phases of the cycle?

The durations and proportions in the table are approximate values given for the scalp; they differ slightly from source to source and vary from person to person.

Anagen: the growth phase

In this phase, the cells of the matrix (the layer of cells at the base of the follicle that produces the hair) divide rapidly, and the hair grows a little every day; scalp hair grows about one centimeter a month. Because the length of this phase varies from person to person, some people’s hair rarely grows past a certain length.

Catagen: the transition phase

Growth signals weaken; cell division in the matrix stops, and so does the production of the pigment that gives the hair its color. The lower part of the follicle shrinks back in a controlled way, and the end of the hair at the root turns into a small, colorless knob. The dermal papilla (the small structure at the base of the follicle that sends growth signals) is drawn upward toward the bulge region, where the stem cells are found. The upper part of the follicle stays in place.

Telogen: the resting phase

The hair no longer grows, but it does not fall out straight away either; it stays anchored in the follicle by the knob at its root. The small white tip you see at the root end of hairs that come out when you comb or wash your hair is often this knob; it does not mean that the follicle is gone.

Exogen and kenogen

These two phases were described relatively recently and are not counted as separate phases in every source. Exogen is the phase in which the resting hair comes loose from the follicle and falls out. The old hair can fall out as the new hair starts to grow, or before it does. If it falls out before the new hair appears, the follicle stays empty for a while; this gap is called kenogen. Under normal circumstances, this gap is short; in genetic hair loss, it happens more often and can last longer.

Why is it normal to lose some hair every day?

At any given time, about 10–15% of hairs are in the resting phase, and these hairs fall out one after another over a few months. That is why losing about 50–100 hairs a day is considered normal for most people.

This number is not the same for everyone. People with long or thick hair may notice shedding more. If you do not wash your hair often, some of the hairs that were going to fall out anyway may come out on wash day, which can make it look as if you are losing more hair on that day.

If the shedding increases noticeably, if your hair is falling out in patches, if your scalp is itchy, red or scaly, or if you notice your hair getting thinner, the cause needs to be looked into. We explain which situations need attention in our article on when hair loss is not normal. We recommend that you see a dermatologist for a diagnosis.

How does the cycle explain hair loss?

Telogen effluvium: many hairs entering the resting phase at the same time

An illness with a high fever, major surgery, giving birth, severe stress, rapid weight loss or certain medicines can push many follicles that are in the growth phase into the resting phase earlier than usual. Conditions such as thyroid problems or iron deficiency may also be linked to similar shedding. Because the resting phase lasts about three months, the shedding usually starts around two to three months after the trigger; this is why most people do not immediately connect the shedding with its cause.

This condition is called telogen effluvium. The hair usually thins across the whole head, without forming patches. Because the follicles are not lost, the hair often recovers within months once the trigger is gone. However, the shedding can sometimes go on for a long time or occur together with another type of hair loss. That is why the diagnosis is made by a doctor during an examination; we do not diagnose anyone remotely. You can find the details in our article on telogen effluvium.

Genetic hair loss: a growth phase that keeps getting shorter

In androgenetic alopecia (hair loss linked to genetic predisposition and hormones), the cycle does not shift for a short time; it changes gradually. In sensitive follicles, the growth phase gets shorter from one cycle to the next, the follicle shrinks (miniaturization), and the hair it produces becomes thinner and thinner. The share of hairs in the resting phase increases, and the gap during which the follicle stays empty may also get longer. Over time, these areas are left with fine, short hairs that do not cover the scalp. In this type of hair loss, what stands out is less the number of hairs shed each day than the hairs becoming finer and the hair thinning out.

How does the cycle explain shedding after a hair transplant?

In a hair transplant, grafts (small pieces of tissue carrying one or more hairs) are usually taken from the donor area at the back and sides of the head and placed in the recipient area. During this time, the grafts spend a while outside the body, cut off from the blood supply. After this stress, a large share of the transplanted follicles enter the resting phase; the hair falls out, but the follicle usually stays in the skin. The procedure can also push some of your own follicles in the surrounding area into the resting phase early.

That is why, in the weeks after the procedure, some of the transplanted hair or the hair around it may fall out temporarily; this is called shock loss. When it happens and to what extent varies from person to person. However, some of the surrounding hairs that had already become thin may not grow back. We cover this topic separately in our article on shock loss.

When the resting phase is over, the follicle enters a new growth phase. New hair usually starts to appear within a few months, at first as fine, sparse hairs; it takes longer for the hair to thicken and for the result to become clear. Because the follicles do not all enter the new cycle at the same time, it is common for the appearance to be uneven in the first months. Both this period and the density achieved vary from person to person; you can find the details in our article on when transplanted hair starts to grow.

Transplanted follicles usually keep the characteristics of the area they were taken from; this is also why hairs transplanted from the scalp to the eyebrows often grow longer than eyebrow hairs. A transplant, however, does not stop the loss of your own hair in areas that were not transplanted; this hair loss may progress over time. After the procedure, the written aftercare instructions we give you always take precedence over general information.

What is still unclear about the cycle?

How the cycle works in general is well understood; however, some questions are still open:

• What exactly triggers the switch from one phase to the next? Signals between the dermal papilla and the stem cells are known to play a role; however, not all the details of how this is regulated have been explained yet.
• Why do the figures vary from source to source? Phase durations and proportions are averages; different values are reported depending on age, the area of the scalp and the measuring method. What share of your follicles is in which phase cannot be measured at home.
• Who will have shedding after a transplant, and how much? It is not possible to predict this precisely in advance, and the exact mechanism of this shedding is not yet fully clear either.
• Can the cycle be influenced from outside? The effect of some medicines used for hair loss is partly linked to this cycle. Only your doctor can determine, after assessing you, whether these medicines are suitable for you; do not start taking them on your own. The evidence on how vitamin supplements, shampoos and supporting procedures affect the cycle, however, is limited.

How do we take the cycle into account during the examination?

When you come to us with hair loss, we keep the cycle in mind and look at the following together:

• When the hair loss started, and whether there was a possible trigger in the few months before, such as a high fever, surgery, giving birth, severe stress or rapid weight loss
• Whether the hair loss is spread across the whole head or concentrated in certain areas
• Differences in thickness between hairs, that is, whether there are follicles that are shrinking; trichoscopy (examining the scalp with a magnifying device) can help with this assessment
• The medicines you take and any health conditions you know of

If the hair loss started recently or its cause is not clear, we want the cause to be clarified before we plan a hair transplant. While heavy shedding is still going on, it is difficult to judge how your hair will look once the shedding settles down. That is why we may postpone the transplant and recommend that you see a dermatologist.

Whether you are suitable for a hair transplant is determined during the examination, and results vary from person to person.