Hair growth is a precisely timed, cyclical event controlled within the hair follicle. This renewal mechanism is genetically predetermined, ensuring a continuous supply of hair as individual strands are replaced over time. The entire cycle involves alternating periods of rapid growth, structural regression, and complete rest before the process begins anew. This highly regulated sequence manages the lifespan of every single hair on the body.
Anagen: The Active Growth Phase
The Anagen phase represents the period of active growth, characterized by rapid cell division within the hair bulb. Cells multiply quickly in the hair matrix, forming the hair shaft and pushing the strand upward and out of the follicle. This is the longest phase of the cycle, accounting for approximately 85% to 90% of all hairs on the scalp at any given time.
The duration of Anagen is highly variable, lasting anywhere from two to seven years for scalp hair, though it is much shorter for body hair. This genetically determined period dictates the maximum potential length a person’s hair can achieve.
Hair typically grows at a rate of about one centimeter per month during this stage. The dermal papilla, a structure at the base of the follicle, supplies the necessary nutrients and blood flow to fuel this rapid proliferation. The longer this active connection remains intact, the longer the hair continues to lengthen.
Catagen: The Transition Phase
The Catagen phase is a brief, controlled transition that signals the end of active growth. Lasting only about two to three weeks, this stage involves structural changes that prepare the follicle for its resting period. Only a small percentage, roughly 1% to 3%, of hairs are typically in the Catagen phase at any moment.
During this short window, the hair follicle shrinks significantly, regressing to about one-sixth of its original size. The active growth process ceases as the hair shaft detaches from the dermal papilla, cutting off its nutrient supply. This detachment involves programmed cell death in the hair matrix cells.
This structural change results in the formation of a “club hair,” characterized by a bulbous, keratinized end. Although no longer actively growing, the hair remains anchored in the follicle during this transition.
Telogen: The Resting and Shedding Phase
The Telogen phase is a state of dormancy where the follicle is completely at rest. This phase typically lasts around three to four months for scalp hair. During this time, the club hair strand is anchored in the follicle but is not growing.
Approximately 5% to 15% of all scalp hairs are in the Telogen phase at any given time. Shedding, which is technically a separate sub-phase called Exogen, occurs when the new Anagen hair begins to emerge from the follicle. The emerging strand pushes the old, resting club hair out of the scalp.
It is considered normal to lose between 50 and 100 Telogen hairs daily as part of this natural renewal process. This continuous, asynchronous cycling ensures that the vast majority of hair remains in the growth phase, maintaining consistent density across the scalp.
How Cycle Disruptions Affect Hair Health
The delicate balance of the hair cycle can be significantly altered by various internal and external stressors, leading to noticeable changes in hair health.
Hormonal and Genetic Factors
Hormonal fluctuations are a major factor, as seen in conditions like androgenetic alopecia. Here, a genetic sensitivity to the hormone dihydrotestosterone (DHT) causes the Anagen phase to shorten progressively. This shortening results in the hair follicle miniaturizing, producing finer and shorter hair over time.
Stress and Nutritional Deficits
Acute physical or psychological shock can force a high percentage of Anagen hairs prematurely into the Catagen and Telogen phases, a condition known as Telogen Effluvium. This can be triggered by events like high fever, major surgery, severe emotional stress, or pregnancy, with noticeable shedding occurring about three months after the initial trigger event. Nutritional deficits can also interrupt the cycle; low levels of iron, protein, or zinc can weaken the follicle and accelerate the shift toward the resting phase.
These disruptions cause hair loss because the ratio of growing to resting hairs is severely skewed. When too many follicles enter the Telogen phase simultaneously, the subsequent shedding becomes diffuse and visible. Addressing the underlying cause, whether it is a nutrient deficiency or chronic stress, is necessary to restore the normal, healthy rhythm of the growth cycle.