Most people only think about the hair growth cycle when something goes wrong with it. A widening part, a thinning crown, a burst of shedding after a stressful month, the slow recognition that the hairline is not where it used to be. Behind every one of those experiences is the same underlying biology: roughly one hundred thousand follicles on the scalp, each one independently cycling through phases of growth, regression, rest, and shedding. Understanding that cycle is the foundation for making sense of pattern hair loss, treatment timelines, and why hair behaves the way it does.
The hair growth cycle is one of the few processes in the body where every individual unit operates on its own clock. A follicle on the temple may be in active growth while its neighbor is shedding. This asynchrony is what allows scalp hair to appear continuous and full, even though every hair on your head is constantly entering and leaving its growth phase. When that asynchrony breaks down, or when the proportions of follicles in each phase shift in the wrong direction, hair density visibly changes.
This guide walks through the four phases of the cycle, the proportions you would expect to find on a healthy scalp, how the cycle changes with age and body region, what androgenetic alopecia does to disrupt it, and how medical treatments work by influencing cycle timing. The goal is not memorization. The goal is a clear mental model that makes everything else about hair loss easier to interpret.
The four phases of the hair growth cycle
Hair biology textbooks describe four distinct phases. Anagen is the active growth phase. Catagen is a short transitional regression. Telogen is the resting phase. Exogen is the actual shedding event, which has historically been treated as a sub-phase of telogen but is increasingly described separately because it is governed by its own signaling.
Anagen: the active growth phase
Anagen is where almost all of the visible work happens. During anagen, the dermal papilla at the base of the follicle is metabolically active, the hair matrix cells are dividing rapidly, and the hair shaft is being produced and pushed upward. On the scalp, anagen typically lasts between two and seven years, with most sources citing an average of three to five years. The length of anagen is the single most important determinant of how long a hair can grow. Someone whose anagen phase is six years long will be able to grow hair past the shoulders. Someone whose anagen is two years long will not.
Anagen length is largely genetically determined, but it is not fixed. It can be shortened by androgens in genetically sensitive follicles, by physiological stress, by certain medications, by nutritional deficiencies, and by chronic inflammation. Treatments that work by extending anagen, including minoxidil, are essentially trying to reverse those shortening forces.
Catagen: regression
Catagen is brief and easy to overlook. It lasts roughly two to three weeks. During catagen the follicle stops producing hair, the lower portion of the follicle involutes, and the hair shaft detaches from its blood supply. The follicle shrinks, but the dermal papilla remains in place, suspended in connective tissue, ready to be re-recruited later. Catagen is a controlled, programmed process and is not associated with hair loss in itself. It is simply the off-ramp from active growth.
Telogen: rest
Telogen is the resting phase, lasting approximately two to four months on the scalp. During telogen the follicle is dormant. The hair is no longer growing but it remains anchored in the follicle, held in place by the club hair structure. From the outside, a telogen hair on the scalp looks indistinguishable from an anagen hair. You cannot tell by looking at a hair whether it is actively growing or simply waiting to be shed.
The proportion of follicles in telogen is one of the most useful diagnostic numbers in dermatology. On a healthy scalp, roughly 10 to 15 percent of follicles are in telogen at any given time. When that proportion rises sharply, often to 25 percent or more, the result is the diffuse shedding seen in telogen effluvium.
Exogen: shedding
Exogen is the active release of the club hair from the follicle. This is what most people think of as "shedding": hairs falling on the pillow, in the shower, or onto a comb. Exogen is normal. Losing 50 to 100 hairs per day is widely cited as a typical baseline, though the actual range varies by individual and by hair length. Importantly, exogen does not necessarily happen at the very end of telogen. A follicle may carry a club hair for some time before releasing it, and a new anagen hair may even begin growing underneath the old one before the old one sheds.
Typical proportions on a healthy scalp
Pulling these numbers together, here is what a roughly healthy scalp looks like in cross-section at any moment:
- About 85 to 90 percent of follicles in anagen
- A very small fraction (often under 1 percent) in catagen
- About 10 to 15 percent in telogen, with some overlap with exogen
These percentages are useful as a frame of reference, not as a diagnostic threshold. Individual variation is significant. A scalp biopsy or trichogram will give a more precise distribution and is something a dermatologist may use when the cause of shedding is unclear.
How the cycle varies by body region
The hair growth cycle is not the same everywhere on the body. Scalp hair has the longest anagen phase, which is why scalp hair grows long. Eyebrow, eyelash, and body hair follicles have much shorter anagen phases, often just a few weeks to a few months, which is why those hairs reach a fixed length and stop. The same biological machinery is at work in every follicle. The difference is the duration that each phase is allowed to last and the local signaling that determines when to switch.
This is also why scalp follicles, beard follicles, and body follicles can react differently to the same hormone. Beard follicles respond to androgens by lengthening anagen and producing thicker, terminal hairs. Genetically sensitive scalp follicles respond to the same androgens by shortening anagen and producing thinner, miniaturized hairs. The hormone is the same. The follicle's interpretation of the signal is opposite. This paradox is at the heart of androgenetic alopecia.
How aging affects the cycle
With age, the anagen phase tends to shorten across the scalp, even in people without a strong genetic pattern of hair loss. Hair shafts produced by older follicles are often slightly thinner, and the synchrony between follicles may degrade somewhat, leading to a more uniform reduction in density rather than a focal pattern. This is sometimes described as "senescent" thinning and is generally distinct from androgenetic alopecia, though the two can coexist and reinforce each other.
The dermal papilla itself can lose cells and signaling capacity over time, which contributes to longer telogen phases and slower re-entry into anagen. This is part of why hair regrowth after a shedding event tends to be slower in older adults than in younger ones, even when the underlying cause has resolved.
How androgenetic alopecia disrupts the cycle
Androgenetic alopecia is, at its core, a disorder of cycle timing. In genetically susceptible follicles, dihydrotestosterone (DHT) binds to androgen receptors in the dermal papilla and triggers a progressive shortening of the anagen phase. Over many cycles, anagen drops from years to months, then weeks. The follicle has less and less time to produce a hair before being switched off, and each successive hair is shorter and thinner than the last. This process is called follicle miniaturization.
The clinical consequences are predictable. The proportion of follicles in telogen rises. The ratio of terminal hairs to vellus-like hairs in affected zones drops. Visible density decreases not because follicles are dying outright, but because the cycle has been so compressed that each follicle now spends more time at rest and less time growing meaningful hair. Eventually, in late-stage stages of the condition, some follicles may be lost entirely, but the early and middle phases of the disease are dominated by cycle disruption rather than follicle death.
This distinction matters clinically. As long as the follicle is still cycling, even at a compressed and miniaturized level, intervention can potentially extend anagen and partially reverse the visible thinning. Once a follicle has been lost, no medical treatment currently available can recreate it.
How medical treatments influence the cycle
The two best-studied medical treatments for pattern hair loss work by acting on the cycle in different ways.
Minoxidil: extending and re-entering anagen
Minoxidil's effects on the hair cycle are well documented. Topical and oral minoxidil both appear to shorten telogen, prolong anagen, and increase the size of the dermal papilla over time. In practical terms, this means follicles spend more time growing and less time resting, and each anagen hair is somewhat thicker than it would have been without treatment. The mechanism is not fully understood but is thought to involve potassium channel opening, increased cutaneous blood flow, and direct effects on follicular signaling pathways.
A well-known phenomenon when starting minoxidil is an initial shedding period, sometimes called "dread shed." This happens because some follicles in late telogen are pushed to release their club hairs as new anagen growth begins underneath. It can be alarming, but it is generally a sign that the medication is engaging the cycle. We discuss this in more detail in why hair sheds when you start treatment.
For a deeper dive on this medication, see how minoxidil treats hair loss and topical versus oral minoxidil.
Finasteride and dutasteride: removing the cycle-shortening signal
Finasteride and dutasteride act upstream, on the hormonal driver of cycle disruption. By inhibiting 5-alpha-reductase, they reduce the conversion of testosterone to DHT, lowering scalp DHT levels and removing much of the pressure that was shortening anagen in androgen-sensitive follicles. Over months, the cycle gradually re-lengthens, miniaturized hairs become more robust, and the proportion of follicles in productive anagen rises.
These two treatments operate on different parts of the cycle, which is why they are often used together. Minoxidil pushes follicles into and through anagen. The 5-alpha-reductase inhibitors keep the cycle from being prematurely shortened. For more on the comparison, see minoxidil versus finasteride and finasteride versus dutasteride.
Why treatment timelines are what they are
The hair growth cycle is also why treatment for pattern hair loss is slow. A miniaturized follicle that has been pushed back toward normal anagen still has to actually grow a hair, and that hair has to surface and lengthen visibly before any change is apparent. Anagen is measured in years, not weeks. Most clinical evaluations of treatment effect are done at three, six, and twelve months for this reason. Earlier than three months, the cycle simply has not had enough time to demonstrate its response.
This biology is why we generally advise patients to expect minimal visible change in the first three months of treatment, modest improvements between three and six months, and the most meaningful changes between six and twelve months. Some patients continue to improve through eighteen months and beyond. We cover the typical arc in how long hair loss treatment takes and in what to expect in the first six months.
It is also why stopping treatment tends to undo gains: removing the medication removes the signal that was holding follicles in productive anagen, and the cycle drifts back toward its untreated baseline over the following months. This is discussed in what happens if you stop treatment.
When cycle disruption suggests something other than pattern loss
Not all cycle disruptions are androgenetic. Sudden, diffuse shedding two to three months after a major life event (a high fever, a surgery, a pregnancy, a period of rapid weight loss) is more characteristic of telogen effluvium, where a synchronized cohort of follicles shifts into telogen at once. This is generally self-limited, with regrowth visible three to six months after the trigger resolves. We cover this pattern in stress and hair loss.
Patchy hair loss with smooth, well-circumscribed bald areas suggests alopecia areata, an autoimmune condition with different cycle dynamics again. Scarring forms of hair loss damage the follicle structure itself and are not primarily cycle disorders. Any of these patterns warrants evaluation by a dermatologist, since the treatments differ substantially.
What to take away
A few practical points are worth holding on to. First, the hair growth cycle is asynchronous: at any moment, follicles are spread across all four phases, which is why scalp hair appears continuous rather than coming in and out in waves. Second, anagen length is the dominant determinant of how long hair can grow and how dense the scalp can appear. Third, androgenetic alopecia is best understood as a progressive shortening of anagen in genetically sensitive follicles, not as the immediate death of those follicles. Fourth, medical treatments work by influencing different parts of the cycle, which is why combining them often provides additive benefit. And fifth, the cycle's slow tempo is the reason treatment timelines are measured in months and years, not days and weeks.
If you are evaluating treatment options or trying to make sense of an unexpected change in your hair, an understanding of the cycle is the most useful single concept you can carry into the conversation. From there, the choices about whether and how to intervene become much clearer.
