Human hair serves various biological purposes beyond aesthetics, including protection from UV radiation and insulation. Its growth is not a continuous process but rather occurs in a series of distinct, recurring phases. Each hair follicle on the body operates independently, undergoing its own cycle of growth, regression, and rest. This cyclical nature ensures a continuous renewal of hair, allowing old hairs to be replaced by new ones. Understanding these cycles provides insight into hair growth and changes.
The Anagen Phase
The anagen phase represents the active growth period for a hair strand. During this time, cells within the hair matrix at the base of the hair follicle rapidly divide, forming new hair cells. These newly formed cells push upward, keratinize, and consolidate to create the hair shaft, which then emerges from the scalp.
For scalp hair, the anagen phase typically lasts between two to seven years, though this duration can vary significantly among individuals due to genetic predispositions. The length of this active growth phase is a primary determinant of how long a person’s hair can naturally grow. At any given moment, approximately 85% to 90% of all hairs on a healthy scalp are actively growing in the anagen phase. Shorter anagen phases are characteristic of hair on other body parts, such as eyelashes or eyebrows, which explains their limited length.
The Catagen Phase
Following the prolonged anagen phase, hair enters the catagen phase, a brief transitional period. This phase signals the end of active hair production and prepares the hair for its resting stage. The hair follicle undergoes a process of involution, meaning it shrinks and detaches from the dermal papilla, its blood supply. The lower part of the hair follicle degenerates, and the hair bulb retreats towards the surface of the scalp.
The catagen phase is relatively short, usually lasting only about two to three weeks. Only a small percentage of hairs, typically around 1% of the total, are in this regressive state at any given time.
The Telogen Phase
The telogen phase is the resting period that follows the catagen phase. The hair shaft, now fully formed and detached from its blood supply, remains in the follicle, held in place by its root sheath. This old hair is often referred to as a “club hair” because of its distinctive root shape.
This resting phase typically lasts for about three months. Approximately 10% to 15% of all scalp hairs are in the telogen phase at any given moment. At the end of this phase, the old hair is naturally shed, either through normal washing or brushing, or it is pushed out by the emergence of a new anagen hair. This shedding is a regular part of the hair growth cycle, making way for new growth.
Factors Affecting Hair Growth
Several internal and external factors can significantly influence the normal progression of hair growth cycles:
- Genetics play a substantial role, determining individual variations in anagen phase duration and overall hair characteristics.
- Hormonal fluctuations, such as those experienced during pregnancy, childbirth, or due to thyroid imbalances, can disrupt cycles, often leading to increased shedding. For instance, a rise in androgens can shorten the anagen phase in genetically predisposed individuals, contributing to pattern hair loss.
- Nutritional deficiencies, particularly in iron, zinc, or certain vitamins like biotin and vitamin D, can impair hair follicle function and lead to reduced growth or increased shedding.
- High levels of psychological or physical stress can prematurely push a significant number of hairs into the telogen phase, resulting in noticeable hair loss several months later.
- The natural aging process also shortens the anagen phase and can reduce hair follicle density.
- Certain medications, including some chemotherapy drugs, anticoagulants, and antidepressants, are known to interfere with hair growth.
- Environmental exposures and physical damage from styling practices can also contribute to hair breakage and impact the perceived health of the hair.