Human hair, a seemingly simple structure, exhibits complex growth patterns. While it might appear that hair grows indefinitely on the scalp but stops at a certain length elsewhere on the body, this overlooks a regulated biological process. Hair growth is not continuous; instead, it is a cyclical phenomenon orchestrated by specialized structures in the skin.
The Hair Follicle and Its Cycle
Hair originates from the hair follicle, a tunnel-like structure extending into the skin. Each hair follicle undergoes repeated cycles of growth and rest throughout its lifespan. This cyclical process ensures hair is constantly renewed, preventing simultaneous shedding of all hair.
The hair growth cycle consists of three primary phases. The anagen phase is the active growth period, where cells in the hair bulb rapidly divide, forming new hair. The duration of this phase is the primary determinant of hair length, with scalp hair typically remaining in anagen for 2 to 7 years.
Following anagen, the catagen phase marks a brief transitional period lasting approximately 2 to 3 weeks. During catagen, hair growth ceases, the follicle shrinks, and the hair detaches from its blood supply.
The final stage is the telogen phase, a resting period that typically lasts 2 to 3 months for scalp hair. During telogen, the old hair remains in the follicle but does not grow. This resting hair, often called a “club hair,” eventually sheds as a new anagen phase begins, pushing it out.
Biological Signals Controlling Hair Growth
The timing and duration of each hair cycle phase are controlled by biological signals. Genetics plays a significant role, as inherited traits influence the maximum potential length and growth patterns of an individual’s hair. This genetic programming determines the typical length of the anagen phase for different hair types across the body.
Hormones also modulate hair follicle activity. Androgens, such as testosterone and dihydrotestosterone (DHT), influence hair growth, promoting or inhibiting it depending on the body area and genetic predisposition. Thyroid hormones and estrogens are additional examples of endocrine signals that affect hair follicle cycling and overall hair health.
Various signaling molecules, including growth factors and cytokines, produced within the hair follicle and surrounding tissues, promote cell proliferation and hair shaft elongation or signal the cessation of growth and entry into the resting phase. Follicular stem cells, located within the hair follicle, are crucial, regenerating the hair follicle during each cycle and initiating new growth.
Why Hair Length Varies on Your Body
The noticeable differences in hair length across various body parts, such as scalp hair versus eyelashes or arm hair, are directly attributable to the varying durations of the anagen phase in follicles located in these regions. While scalp hair has an anagen phase lasting several years, allowing it to grow long, eyebrow and eyelash hairs have a much shorter anagen phase, typically only a few months.
This variation is influenced by localized genetic programming, hormonal sensitivities, and the specific composition of growth factors in the skin. For instance, the anagen phase for arm and leg hair is shorter, lasting only weeks to a few months, which prevents it from growing to the same lengths as scalp hair. Each hair follicle operates independently, ensuring that not all hairs are in the same phase of the cycle simultaneously, leading to continuous, staggered hair growth and shedding.
What Happens When the Cycle is Disrupted
Disruptions to the hair growth cycle can lead to various hair conditions, including hair loss or changes in hair texture. Androgenetic alopecia, commonly known as pattern baldness, occurs when genetic and hormonal factors, particularly dihydrotestosterone (DHT), shorten the anagen phase. This shortening results in hair follicles producing finer, shorter hairs, and eventually ceasing hair production.
Another common disruption is telogen effluvium, a temporary condition characterized by increased hair shedding. This occurs when many hairs prematurely enter the telogen (resting) phase due to a physiological stressor, such as severe illness, hormonal changes, or nutritional deficiencies. These instances highlight how temporary imbalances can impact the hair cycle.