During the catagen phase, hair stops growing and the lower portion of the follicle shrinks dramatically through programmed cell death. This transition period lasts about two weeks and serves as the bridge between active growth (anagen) and the resting stage (telogen). It’s one of the shortest and least understood phases of the hair cycle, but what happens during these two weeks determines whether a hair strand is shed smoothly or prematurely.
How Long Catagen Lasts and How Common It Is
Catagen is brief, lasting roughly two weeks compared to the two to seven years of active growth that precedes it. Because it’s so short, only a small fraction of your scalp hair is in this phase at any given moment. The traditional teaching has been that about 1% to 2% of scalp follicles are in catagen at once, with 85% to 90% in anagen and 10% to 15% in telogen.
More recent research challenges those numbers. A 2024 study using detailed tissue analysis found that catagen follicles actually made up about 7.5% of scalp follicles in the men studied, while telogen follicles accounted for only 3.5%. That’s a significant reversal of the old assumption that telogen follicles always outnumber catagen ones. The discrepancy likely comes from earlier methods that misclassified some catagen follicles or relied on less precise counting techniques.
Growth Slows, Then Stops
The most visible characteristic of catagen hair is that it barely grows. Anagen hair elongates at roughly 1 millimeter every three days. Catagen hair shows only slight elongation over the same period, and telogen hair doesn’t grow at all. This slowdown is one of the ways researchers distinguish catagen hairs from anagen hairs using sequential photographs of the scalp, a technique called phototrichography.
From the outside, a catagen hair looks the same as any other strand on your head. The changes are all happening beneath the skin’s surface, inside the follicle itself.
The Follicle Shrinks Through Cell Death
The defining event of catagen is the controlled destruction of the lower follicle. Cells in the hair bulb and surrounding structures undergo apoptosis, a form of programmed cell death where cells essentially dismantle themselves in an orderly way. This isn’t damage or disease. It’s a built-in process that the follicle initiates on schedule.
The apoptosis doesn’t happen everywhere equally. Dying cells appear in the regressing lower follicle, the inner root sheath (which normally anchors the growing hair), and even near the bulge region higher up in the follicle. One structure is notably spared: the dermal papilla, a tiny cluster of cells at the base of the follicle that acts as the command center for hair growth. Research has confirmed that apoptotic cells never appear in the dermal papilla during catagen. Protecting this structure is essential because the dermal papilla contains the signals needed to restart growth in the next cycle.
The Dermal Papilla Moves Upward
As the lower follicle degrades, something important happens to the dermal papilla. Rather than being destroyed along with the surrounding tissue, it gets drawn upward as the follicle contracts. By the end of catagen, the dermal papilla sits just beneath the permanent upper portion of the follicle, next to a small cluster of cells called the secondary germ. This repositioning is critical. The secondary germ and the dermal papilla will interact to regenerate the follicle when the next growth phase begins.
Think of it like a building being partially demolished while the architects are safely relocated to the top floor, ready to oversee reconstruction.
A Club Hair Begins to Form
During catagen, the base of the hair strand transforms. In the anagen phase, the root is soft, pliable, and deeply embedded in the follicle bulb. As catagen progresses and the bulb degrades, the root hardens and develops a rounded, club-shaped tip. This is what gives “club hair” its name. The club shape is essentially a keratinized cap that anchors the strand loosely in the follicle without any active connection to blood supply or growth signals. A fully formed club hair will eventually be shed during or after the telogen phase, often pushed out when a new anagen hair starts growing beneath it.
What Triggers the Transition
The shift from active growth into catagen isn’t random. It’s driven by molecular signals within and around the follicle. Multiple signaling pathways work in parallel to flip the switch. One well-studied trigger is a growth factor called FGF5 (fibroblast growth factor 5). Mice that lack this protein have abnormally long hair because their follicles stay in anagen longer than usual, which tells researchers that FGF5 normally helps push follicles into catagen on schedule.
Other signaling molecules, including parathyroid hormone-related protein, also regulate this transition through independent pathways. The fact that multiple systems control catagen entry helps explain why hair growth timing is so robust. If one signal is disrupted, others can compensate. Inside the follicle, the balance between pro-survival and pro-death proteins shifts during catagen. Proteins that protect cells from dying get dialed down, while those that promote cell death increase, tipping the scales toward the controlled regression that defines this phase.
Why Catagen Matters for Hair Loss
In a healthy scalp, catagen is a brief, unremarkable transition. But when something disrupts the hair cycle, follicles can be pushed into catagen prematurely. Certain chemotherapy drugs, for instance, trigger massive cell death in the hair bulb that mimics an accelerated version of catagen, causing widespread shedding. Stress-related hair loss (telogen effluvium) also involves follicles exiting anagen earlier than they should, passing through catagen, and entering the resting phase all at once.
Understanding catagen also matters for conditions like androgenetic alopecia (pattern baldness), where the growth phase gets progressively shorter with each cycle. As anagen shrinks, the relative time spent in catagen and telogen increases, and hairs become thinner and shorter with each generation. The recent finding that catagen follicles may be more prevalent on the scalp than previously thought could reshape how researchers measure and interpret hair loss in clinical settings.