Hair growth is not a continuous process. Instead, it follows a carefully orchestrated cycle with distinct phases, each governing the life of an individual hair strand. This biological reality explains why hair doesn’t simply grow forever.
The Phases of Hair Growth
Hair growth proceeds through four main phases: anagen, catagen, telogen, and exogen. Each hair follicle operates independently, meaning different hairs on your head can be in different stages simultaneously. This asynchronous cycling prevents all hair from shedding at once.
The anagen phase is the active growth period, where hair follicles produce new hair cells. For scalp hair, this phase typically lasts between two and eight years, and it determines the maximum length a hair can achieve. Approximately 85% to 90% of the hairs on your head are in this active growth phase at any given time. Hair generally grows about half an inch (1.25 cm) per month during this stage.
Hair then enters the catagen phase, a short transitional period lasting about two to three weeks. During this time, hair growth slows, and the hair follicle shrinks, detaching from its blood supply. Only a small percentage, around 1% to 3%, of scalp hairs are in this phase at any given moment.
The telogen phase is a resting period for the hair follicle, lasting approximately two to three months. The hair remains in the follicle but is not actively growing. About 10% to 15% of scalp hairs are typically in this resting stage.
Finally, the exogen phase is when the old hair sheds, making way for new hair to begin its anagen phase. This shedding is a normal part of the cycle, with most individuals losing between 50 to 100 hairs daily. The exogen phase can last for several months.
Factors Affecting Hair Growth
Several internal and external factors influence the hair growth cycle, affecting how quickly hair grows, its density, and its overall health. Genetics play a significant role, determining hair texture, density, and anagen phase duration. Inherited traits influence potential hair length and the likelihood of conditions like pattern baldness.
Nutrition is another important factor, as hair follicles require specific nutrients to function optimally. A balanced diet with sufficient vitamins (biotin, D, C) and minerals (iron, zinc) supports healthy hair growth. Deficiencies in these nutrients can disrupt the hair cycle, potentially leading to thinning or loss.
Hormonal fluctuations also impact hair growth. Changes associated with pregnancy, childbirth, menopause, or thyroid imbalances can alter the hair cycle, leading to increased shedding or altered growth patterns. Certain hormones can cause hair follicles to shrink, shortening the anagen phase.
Age naturally influences the hair growth cycle, often leading to a shorter anagen phase and decreased hair density. Stress can trigger temporary hair shedding, known as telogen effluvium, where hairs prematurely enter the resting phase. Health conditions and medications can also affect the hair growth cycle, leading to various forms of hair loss.
Why Hair Length Has a Limit
The primary reason hair does not grow indefinitely is the genetically determined duration of its anagen, or active growth, phase. This inherent biological limit means that each hair strand can only reach a certain maximum length before its growth cycle concludes. This biological programming varies for different types of hair on the body; for example, the anagen phase for scalp hair is much longer than for eyebrow or body hair, explaining why head hair can grow considerably longer.