Hair is a keratinous filament produced by the hair follicle, a specialized mini-organ embedded in the skin. The growth of this filament is not a continuous process but rather an intricate, highly regulated system of predictable, repeated cycles that occur independently across the scalp and body. Understanding this cyclical process is foundational to grasping what determines hair length, density, and overall health. The continuous rotation of these phases ensures that a stable number of hairs remain on the head at any given time, preventing mass, simultaneous shedding.
The Three Phases of Hair Growth
The hair growth cycle is divided into three primary stages: the growth phase (Anagen), the transition phase (Catagen), and the resting phase (Telogen). The Anagen phase is the growth engine, representing the longest stage of the cycle and determining the maximum length a hair can achieve. During Anagen, cells in the hair bulb divide rapidly, creating the hair shaft at a rate of approximately 0.3 to 0.4 millimeters per day. For scalp hair, this active growth period typically lasts between two and seven years. At any given time, roughly 85-90% of the hairs on the scalp are actively engaged in this phase.
The Catagen phase is a brief, transitional period lasting only about two to three weeks that signals the end of active growth. Cell division ceases completely, and the lower part of the follicle begins to regress, detaching from the dermal papilla. The follicle shrinks to about one-sixth of its former length, preparing for the upcoming dormant period.
Following the Catagen phase, the follicle enters the Telogen phase, a period of rest lasting approximately two to four months. The hair remains fully formed but dormant, held in place as a “club hair” at the base of the inactive follicle. The final stage, sometimes referred to as Exogen, is the active shedding of this club hair, which occurs when a new Anagen hair begins to grow beneath it and pushes the old hair out. This mechanism ensures the cycle renews itself, with a healthy scalp typically shedding 50 to 100 hairs daily as part of this natural turnover.
Internal and External Regulators of the Cycle
The precise timing and duration of these phases are governed by a complex interplay of genetic programming and environmental signals. Genetics serves as the primary blueprint, establishing the fundamental length of the Anagen phase, which is why leg hair cycles are significantly shorter than those on the scalp. This genetic code also dictates the sensitivity of the hair follicles to various molecular signals, particularly hormones.
Hormones act as powerful internal messengers that signal the transition between phases. Androgens, such as dihydrotestosterone (DHT), shorten the Anagen phase in genetically susceptible follicles, while estrogen tends to prolong it, contributing to fuller hair during pregnancy. Thyroid hormones regulate the metabolic activity of the follicle; an imbalance can cause a premature shift from Anagen to Telogen, leading to diffuse thinning. Chronic elevation of the stress hormone cortisol can also interfere with signaling pathways, pushing follicles toward early rest.
External factors like nutrition and age also modulate the cycle’s performance. The high metabolic demands of the Anagen phase require a steady supply of nutrients, making hair growth sensitive to dietary deficiencies. Insufficient intake of key micronutrients like iron, zinc, and Vitamin D, or a lack of protein, can disrupt the follicle’s ability to sustain active growth. Age naturally shortens the Anagen phase over time, resulting in a reduction in maximum hair length and overall density.
When the Cycle Fails: Common Disruptions
When the regulatory mechanisms that control the hair cycle malfunction, the result is often noticeable hair loss or changes in hair density. One common failure involves the drastic shortening of the Anagen phase, a process known as follicle miniaturization.
Follicle Miniaturization
This is the hallmark of Androgenetic Alopecia, or pattern hair loss, driven by a genetic hypersensitivity to androgens. Exposure to androgens causes the growth phase to become progressively shorter with each cycle. This results in the production of thinner, shorter, and finer hairs until the follicle eventually becomes dormant.
Telogen Effluvium
Another significant disruption involves the mass synchronization of the cycle, seen in Telogen Effluvium. This occurs when a major physical or emotional stressor, such as a severe illness, surgery, or extreme dieting, abruptly forces a large proportion of growing hairs into the Catagen and Telogen phases simultaneously. Because the Telogen phase lasts approximately two to four months, the resulting mass shedding is delayed, appearing as a sudden, diffuse loss of hair months after the initial triggering event. This disruption is typically temporary, and the cycle usually resets once the underlying stressor is resolved.