The Hair Cycle Clock: What New Research Reveals About What Actually Controls Anagen, Catagen, and Telogen

The hair cycle is one of the first things taught in trichology training. Growth phase, regression phase, resting phase. Most follicles are in anagen — that's why your clients have full heads of hair most of the time. A small number are in telogen — that's why they shed around 100 hairs a day. Catagen is brief, maybe two to three weeks, just the transition between the two.

What those introductory descriptions leave out is the question that actually matters clinically: what is the follicle using to keep time? What molecular signals decide when anagen ends, how long telogen lasts, and when the cycle restarts? For decades, the honest answer was that we didn't fully know. We understood the phases well. We understood them far less well as a system with actual timekeeping logic.

That's changing. A 2025 Nature Communications study identified a chromatin-level switch that directly controls quiescence in hair follicle stem cells. A separate 2024 study challenged the standard textbook percentages for catagen and telogen, suggesting that the transition phase is more active in human scalp than commonly assumed. And an accumulating body of research is clarifying how the WNT, BMP, FGF, and TGF-β pathways don't just participate in the hair cycle — they are the timekeeping mechanism, operating as an integrated antagonistic network. Understanding that network is foundational to understanding why cycling disorders happen and what might actually correct them.

The Phases, Revisited with Actual Mechanism

Anagen is the growth phase. In human scalp follicles it typically lasts two to six years, though this varies considerably by individual and scalp region. During anagen, hair follicle stem cells (HFSCs) in the bulge and secondary hair germ are activated, proliferate, and give rise to the transit-amplifying cells that form the hair matrix, which in turn produces the hair shaft. The dermal papilla — the cluster of specialized fibroblasts at the base of the follicle — acts as a signaling hub throughout, releasing factors that sustain the growth program.

Catagen is the regression phase. It lasts roughly two to three weeks. During catagen, the lower follicle undergoes controlled apoptosis. The hair matrix cells die back. The dermal papilla condenses and migrates upward toward the bulge region. The hair shaft, now disconnected from its blood supply, becomes a club hair anchored only by the epithelial strand still connecting it to the follicle remnant above.

Telogen is the resting phase. In human scalp it typically lasts two to four months, though again with significant individual variation. During telogen, the follicle is quiescent. HFSCs are not dividing. The dermal papilla sits close to the bulge, where it can directly interact with the stem cells when the time comes to restart. Telogen ends with exogen — the shedding of the club hair — followed by the initiation of the next anagen.

What controls the duration of each phase, and what controls the timing of transitions between them, is where the molecular biology gets genuinely interesting.

Challenging the Textbook Numbers

The standard teaching is that at any given moment, 85–90% of scalp follicles are in anagen, 10–15% are in telogen, and 1–2% are in catagen. Those numbers have been cited in textbooks for decades. A 2024 study by Francisco Jimenez and Majid Alam, published in Experimental Dermatology, set out to actually examine them in excised human tissue — and found they don't hold up.

The researchers analyzed hair follicles removed during routine hair transplantation procedures in 14 Caucasian males, using ex vivo stereomicroscopy to assess each follicle's cycle stage directly. Their finding: catagen follicles were present at 7.5% and telogen follicles at 3.5% — meaning the catagen percentage was substantially higher than the textbook figure, and the telogen percentage was substantially lower.

The authors concluded that the percentage of catagen follicles is clearly underestimated in the current literature, likely because catagen follicles are easier to misclassify — both in histological cross-sections and in trichoscopy — than anagen or telogen follicles. If their data is confirmed by larger studies, it would mean that the transition phase is far more present in the human scalp at any given moment than has been assumed, and that models of the hair cycle based on the old percentages need to be revised.

The Molecular Clock: BMP, WNT, and the Switching Mechanism

The most important conceptual advance in hair cycle biology over the past decade is understanding that the transitions between phases are not simply triggered by a single signal. They emerge from an antagonistic signaling network — competing pathways that push against each other until the balance tips decisively in one direction. The primary players are BMP, WNT, FGF, and TGF-β, and understanding what each one does, when, and why, is the key to understanding virtually every hair cycling disorder you'll encounter in practice.

A 2025 comprehensive review in Stem Cell Research & Therapy maps the signaling crosstalk that governs the hair cycle in detail. The BMP-WNT axis is central to understanding what keeps a follicle in telogen and what releases it into anagen.

BMP signaling maintains quiescence. During telogen, bone morphogenetic proteins are produced by multiple sources: the follicle itself, surrounding dermal fibroblasts, adipocytes, and the dermal papilla. This sustained BMP signaling suppresses HFSC proliferation and maintains the stem cells in a dormant state. BMP4 and BMP6, along with FGF18 from the same fibroblast populations, work together to keep the refractory telogen phase in place. This is sometimes described as the "inhibitory macroenvironment" — the surrounding tissue actively suppressing hair cycle reactivation.

WNT signaling activates the cycle. As telogen progresses from refractory to competent phase, BMP signaling begins to fall. WNT ligands increase. This shift is partially autonomous — the follicle's own cells change their signaling — and partially driven by the dermal environment changing its output. When WNT ligands engage Frizzled receptors, the destruction complex is inhibited, β-catenin stabilizes and translocates to the nucleus, and the transcriptional program for HFSC activation begins. Proliferation follows. Anagen begins.

FGF18 acts as a timing component within telogen. FGF18 is expressed by hair follicle stem cells throughout the telogen phase and synergizes with BMP to maintain refractivity. When FGF18 signaling was conditionally knocked out in mouse models, telogen became dramatically shortened and hair cycles accelerated — supporting FGF18's role not just in sustaining quiescence but in determining how long quiescence lasts. It is one of the clearest pieces of evidence that the hair cycle has actual timekeeping molecules, not just on/off switches.

What the Bellani 2025 review makes clear is that these pathways don't act sequentially — they act simultaneously and in tension, with the overall balance shifting as the follicle progresses through its cycle. Crosstalk between WNT-BMP and Shh-Notch is essential for the temporal and spatial coordination of cycling. Disrupting any one arm of this network doesn't simply slow or stop one phase; it throws off the entire timing system.

The Chromatin Switch: A 2025 Discovery That Changes the Picture

The signaling pathways described above operate at the cell surface and in the cytoplasm. But how do those signals actually get translated into sustained changes in stem cell behavior? What keeps an HFSC in a non-dividing state through weeks or months of telogen, even as molecular signals fluctuate around it? A December 2025 paper in Nature Communications from Pooja Flora and colleagues identified a key piece of that answer: a histone modification called H2AK119 monoubiquitination, or H2AK119ub.

H2AK119ub is a repressive histone mark — it silences gene expression at the chromatin level. What the Flora team found is that H2AK119ub functions as a dynamic molecular switch in HFSCs, directly linking inhibitory FGF signals to the quiescent transcriptional state. When FGF signaling (including the inhibitory FGF18 discussed above) is active, H2AK119ub levels are high, and the proliferation-promoting transcriptional program is repressed. The stem cell stays dormant. When FGF signaling falls, H2AK119ub levels drop, the repression lifts, and the stem cell is released toward activation.

This is mechanistically significant because it explains how the cell maintains its quiescent state persistently, not just moment to moment, but across the weeks of telogen. The chromatin modification provides a kind of molecular memory for the quiescent state — it isn't continuously reinduced by every signaling fluctuation, it persists until specific conditions actively remove it.

The Circadian Clock and the Hair Cycle

Beyond the molecular antagonism of BMP and WNT, and beyond chromatin-level memory of quiescence, there is a third timekeeping system operating in the hair follicle: the circadian clock. This has been documented since foundational work published in PLOS Genetics in 2009, but the implications are still underappreciated in clinical trichology.

Core circadian clock genes — CLOCK, BMAL1, Period1 — are expressed in hair follicle cells in a rhythmic, cycling pattern. In organ-cultured human hair follicles, this research demonstrated circadian oscillations in clock gene expression, and showed that Period1 expression is hair cycle dependent, not merely time-of-day dependent. The two systems — the 24-hour circadian clock and the multi-month hair cycle clock — are genuinely interconnected in the same tissue.

Mouse studies using Clock and Bmal1 mutant animals are particularly informative. In these models, deleting the core clock genes delays anagen progression. The secondary hair germ cells — the precursor population that initiates the next growth cycle — show reduced phosphorylated Rb and a lack of mitotic cells in the Clock/Bmal1 mutants. The circadian machinery is not just running in the background of the follicle; it is directly regulating the cell cycle transitions that move stem cells toward activation.

BMAL1 specifically regulates HFSC quiescence by modulating stem cell regulatory genes in an oscillatory manner. The result is that the dormant bulge contains heterogeneous populations of HFSCs: some are at a phase of their circadian cycle that predisposes them to activation, others are at a phase that doesn't. This heterogeneity is functional — it means that not all stem cells become activated simultaneously when anagen is triggered, which is probably important for avoiding the kind of synchronized, exhausting over-activation that the H2AK119ub loss experiments produced.

When the Clock Gets Disrupted: Stress, Cortisol, and Telogen Effluvium

Understanding what normally controls the hair cycle makes the mechanism of disruption clearer. Telogen effluvium — the premature, diffuse shift of follicles into the telogen phase resulting in increased shedding — is typically described as a response to a triggering stressor: illness, surgery, significant weight loss, nutritional deficiency, postpartum hormonal change. But the molecular pathway through which stress actually disrupts the cycle involves several of the same signaling systems discussed above.

Chronic psychological stress activates the hypothalamic-pituitary-adrenal axis, elevating cortisol and corticotropin-releasing hormone. CRH is not just a systemic stress hormone; it is also expressed locally by perifollicular nerve fibers and by immune cells near the follicle. CRH and its receptor are present in the follicle itself, and their activation promotes a shift toward the catagen/telogen program by modulating the local cytokine environment. Substance P, released by sensory neurons under stress, further amplifies this effect by promoting mast cell degranulation near the follicle, releasing pro-inflammatory mediators that push follicles out of anagen.

Oxidative stress compounds the effect. Chronic cortisol elevation impairs mitochondrial function in follicular cells and increases reactive oxygen species. This oxidative load interferes with the WNT/β-catenin and TGF-β pathways that normally coordinate the telogen-to-anagen transition. The molecular clock system is also sensitive to oxidative stress and cortisol, with chronic HPA activation documented to impair BMAL1 function.

A 2025 review in JAAD Reviews synthesizing the psychological stress and hair loss literature confirmed that the pathways linking stress to premature telogen entry involve the HPA axis, neuropeptide release, and downstream inflammatory signaling acting together — not a single trigger but a cascade that converges on the same signaling environment that the BMP-WNT network depends on for normal function.

Therapeutic Implications: Where the Research Is Pointing

Understanding the molecular clock opens specific therapeutic targets. Some of these have clinical-stage evidence. Most are still in preclinical development. But knowing the mechanism changes the clinical conversation.

WNT activation. Small molecules that activate WNT signaling — including CHIR99021 (a GSK-3β inhibitor) and valproic acid (which has WNT-activating properties among others) — have been investigated in AGA models. The challenge is that indiscriminate WNT activation isn't safe; constitutively active β-catenin in follicles can drive pilomatricoma formation. Targeted, controlled WNT activation that mimics the natural competent telogen-to-anagen transition is the goal. This remains an active area of research.

BMP inhibition. Noggin mimetics and BMP-neutralizing antibodies can theoretically accelerate anagen onset by lowering the inhibitory barrier. Noggin itself is naturally produced by dermal papilla cells during the competent telogen phase and contributes to the BMP decline that allows WNT to take over. Exogenous Noggin peptides have shown hair growth promotion in mouse models. Human trials are limited.

FGF2 for cycle promotion. Basic FGF (FGF2) is one of the activating FGFs that promotes follicle cycle re-entry. A Phase I clinical trial evaluating intradermal FGF2 injections in androgenetic alopecia patients reported that 86% of participants showed a decrease in fine hairs and 80% exhibited an increase in terminal hairs. This is early data, but it's human data, and it aligns mechanistically with what the FGF signaling literature predicts.

Sleep and circadian entrainment as non-pharmacological levers. This might sound like lifestyle advice, but it has direct mechanistic grounding. Restoring consistent sleep-wake cycles and light exposure normalizes BMAL1 function. For patients with diffuse shedding in the context of circadian disruption, addressing sleep quality is not just holistic noise — it is targeting one of the identified timekeeping systems for the hair cycle.