The human body is covered in hair, but its length varies dramatically, from long scalp strands to nearly invisible fuzz on the arm. This difference results from a precisely timed biological clock within each hair follicle. The mechanism that dictates when a hair stops growing is a genetically programmed, cyclical process that governs the life span of every individual hair. This timing explains why hair does not grow indefinitely, but instead reaches a predetermined maximum length before shedding and starting anew.
The Three Phases of Hair Growth
Hair growth is a continuous cycle of renewal divided into three main phases: anagen, catagen, and telogen. The cycle ensures that old hair is shed and replaced, preventing temporary baldness because each follicle operates on its own independent schedule.
The anagen phase is the most active and longest stage, where hair root cells divide rapidly to form the hair shaft, which grows continuously at a rate of approximately one centimeter per month. For scalp hair, about 85% to 90% of follicles are in this active growth state at any given time. The duration of this phase is the primary determinant of the hair’s maximum potential length.
The catagen phase is a brief transitional period that signals the end of active growth, typically lasting only two to three weeks. During this stage, the hair follicle shrinks to about one-sixth of its original size and detaches from its blood supply. This regression forms a fully keratinized “club hair” at the base, ready to be shed.
Following the transitional stage is the telogen phase, a resting period that lasts for about two to four months. The hair is not actively growing during this time but remains anchored in the follicle until it is naturally pushed out by the growth of a new hair. The final shedding of the old hair is sometimes classified as the exogen phase.
The Genetic Programming of Hair Length
Hair stops growing at different lengths across the body due to the genetically programmed duration of the anagen phase for each specific follicle location. The maximum length a hair can achieve is determined by how long its follicle is instructed to remain in the active growth state. This genetic blueprint separates long scalp hair from short body hair.
Scalp hair follicles are programmed for an exceptionally long anagen phase, typically lasting between two and seven years, allowing hair to grow over a meter in length in some individuals. This extended growth period is responsible for the long, thick, and pigmented terminal hair found on the head.
In contrast, hair follicles on areas like the arms, legs, and eyebrows are programmed for a much shorter anagen phase, often lasting only a few months. This brief growth window limits the hair to short, fine, and often colorless vellus hair. For example, the anagen phase for an eyebrow hair may only last four to seven months, explaining why eyebrows never grow as long as scalp hair.
Hormonal and Molecular Signals That Control the Cycle
While genetics sets the maximum timer for the anagen phase, the actual transition between phases is executed by a complex network of chemical inputs and molecular signals. The hair cycle is regulated by various signaling pathways, including the WNT pathway, which promotes the active growth phase. The WNT signaling protein is highly expressed during anagen, decreases during catagen, and becomes inactive during telogen.
Growth factors and cytokines are also involved in initiating and maintaining the growth phase. For instance, insulin-like growth factor-I (IGF-1) promotes hair follicle growth and is expressed in the dermal papilla cells that regulate the hair cycle. Conversely, inhibitory signals, such as transforming growth factor-beta (TGF- \(\beta\)), suppress hair growth and promote the regression of the follicle, triggering the catagen phase.
Hormones, particularly androgens like dihydrotestosterone (DHT), are powerful modulators of the cycle, especially in genetically susceptible areas. DHT binds to androgen receptors in the hair follicles, causing them to shrink and progressively shorten the anagen phase in a process known as miniaturization. This disruption is the underlying mechanism for pattern baldness, where the hair grows back finer and shorter with each successive cycle until growth eventually stops. The regulation of the hair growth cycle is a finely tuned process that is sensitive to internal factors like hormones, stress, and nutrition.