Slow hair growth is a common concern, often leading people to believe their hair is not growing. The average rate of growth for human hair is approximately half an inch, or about 1.25 centimeters, per month. When individuals perceive their hair as growing slowly, the reason is typically rooted in two distinct categories: biological disruptions to the hair growth cycle or external damage that causes the hair shaft to break off.
Understanding the Hair Growth Cycle
Hair growth is a repeating, cyclical process that occurs in three main phases: anagen, catagen, and telogen. The anagen phase is the active growth period where cells in the hair follicle divide rapidly, elongating the hair shaft. This phase is the longest, typically lasting between two and seven years, and its duration determines the maximum length a hair can potentially reach.
The catagen phase is a brief transitional stage, lasting only about two to three weeks. During this time, the hair follicle shrinks, and growth ceases as the hair separates from the blood supply. The telogen phase is the resting period, which generally lasts around three months. The hair remains anchored in the follicle before it is shed during the exogen phase, allowing the cycle to begin anew.
Systemic Causes That Shorten Growth Phases
A slow growth rate is often the result of internal factors that disrupt the hair cycle by prematurely shortening the anagen phase. Genetics determine the maximum duration of the anagen phase, explaining why some people can grow their hair much longer than others. Furthermore, as a person ages, the hair growth cycle naturally slows down, leading to a shorter growth phase and reduced hair density over time.
Hormonal imbalances are a significant systemic cause, as key hormones regulate the transition between hair cycle stages. For example, androgens like dihydrotestosterone (DHT) can shorten the anagen phase and miniaturize hair follicles in those genetically predisposed to pattern baldness. Thyroid hormones are also integral; both an underactive or overactive thyroid can disrupt the cycle, resulting in diffuse thinning and brittle hair.
Stress hormones, particularly elevated cortisol, can force a large number of hairs to enter the telogen (resting) phase prematurely, a condition known as telogen effluvium. This shift causes noticeable shedding months after the stressful event, shortening the hair’s lifespan.
The hair follicle requires energy and building blocks, making nutritional deficiencies a direct cause of slowed cell division. Deficiencies in iron, which transports oxygen to the follicle, can deprive the hair root of resources for healthy growth. Inadequate levels of Vitamin D and zinc can disrupt the hair follicle cycling process, leading to increased shedding and a temporary halt in growth. Since hair is composed primarily of the protein keratin, a lack of sufficient dietary protein can also weaken the hair structure and limit the availability of amino acids needed for cell production.
Physical Damage Masking True Growth
In many cases, the hair follicle produces hair at a normal rate, but length is not retained because the hair shaft breaks off near the ends. This is often the result of mechanical stress, which weakens the hair structure over time. Aggressive habits like rough brushing, friction towel-drying, or wearing tight hairstyles can cause the hair to snap along the shaft.
The application of high heat, such as from flat irons or curling wands, can denature the hair’s core protein, keratin, and remove moisture. Heating hair above 60 degrees Celsius can cause permanent damage, leading to brittleness and split ends that inevitably break off.
Chemical treatments, including bleaching, perms, and relaxers, alter the hair’s keratin bonds to change its texture. These chemical alterations leave the hair more porous and less elastic, making it highly susceptible to breakage.
Environmental factors, such as prolonged exposure to ultraviolet (UV) radiation, can degrade the outer protective layer of the hair. When the rate of hair breakage at the ends equals the rate of new growth at the root, the hair appears to stop growing altogether, masking the true biological growth rate.