How does hair grow? The answer lies far beneath the surface in the microscopic world of hair follicles, hormones, and cellular activity. While we may take our hair for granted, its growth is anything but ordinary.
Every strand of hair is a tiny masterpiece, shaped by biology, environment, and care. The growth process itself is a perfect balance of cellular mechanics, hormone regulation, and external factors such as diet and stress.
By understanding the science behind hair growth and how to encourage healthier hair, we can gain a new level of appreciation for what happens just beneath our scalp every day.
The Structure of Hair
Hair is more than just what we see on the surface—it’s a complex structure that extends deep into the scalp. Each strand of hair consists of multiple layers, with both visible and hidden parts that impact growth and health.
The hair shaft is the portion we see, while the root is embedded in the scalp. The hair follicle, a small but powerful structure, anchors the hair and controls its growth cycle.
Hair Shaft and Hair Root
The hair shaft is the visible portion of the hair that extends above the scalp.
It consists of three layers: the medulla (innermost core), the cortex (middle layer responsible for strength and color), and the cuticle (outer protective layer). The shaft is made of keratin, a tough protein that gives hair its structure.
Beneath the surface, the hair root is embedded within the follicle and surrounded by the dermal papilla, which supplies nutrients to support growth. The root contains actively dividing cells that produce new hair, pushing older cells upward.
Unlike the shaft, the root is living tissue and is directly influenced by factors such as blood supply, hormones, and overall health.
The Role of Hair Follicles
Hair follicles are tiny tunnel-like structures in the scalp that house the hair roots. They serve as the foundation for hair growth and are responsible for regulating the hair cycle.
Each follicle is connected to sebaceous glands that produce sebum, a natural oil that conditions the hair and scalp.
The base of the follicle, called the hair bulb, contains matrix cells that divide rapidly to create new hair cells. Surrounding the bulb is the dermal papilla, a structure rich in blood vessels that supply oxygen and nutrients.
The hair follicle itself is surrounded by a network of sensory nerves that detect and respond to touch, temperature, and pain. Follicles also contain tiny muscles called arrector pili, which cause hair to stand up when stimulated.
The health and function of hair follicles determine hair thickness, length, and overall vitality.
The Hair Growth Cycle
Hair doesn’t grow all at once or at the same rate. Instead, each hair follicle operates independently, cycling through different growth stages at different times.
This independent function prevents all your hair from shedding at once, ensuring a continuous and natural appearance.
The hair growth cycle consists of four key phases: anagen (growth), catagen (transition), telogen (resting), and exogen (shedding). Each phase helps determine how long, thick, and healthy hair becomes.
Anagen Phase: The Growth Stage
The anagen phase is the most active stage of hair growth and can last anywhere from two to seven years. During this phase, cells in the hair bulb divide rapidly, producing new keratinized cells that push older ones upward, forming the hair shaft.
The longer a follicle stays in the anagen phase, the longer the hair can grow. Genetics help to determine the duration of the anagen phase, which explains why some people can grow extremely long hair while others cannot.
At any given time, about 85-90% of scalp hairs are in the anagen phase. Proper nutrition, healthy blood circulation, and hormonal balance support this stage and help hair grow strong and resilient.
Catagen Phase: The Transition Stage
The catagen phase is a brief transitional period that lasts about two to three weeks.
During this phase, cell division in the hair bulb slows, and the follicle begins to shrink. The hair detaches from the blood supply, signaling the end of its active growth. Although the hair remains anchored in the follicle, it is no longer growing.
Only about 1% of scalp hairs are in the catagen phase at any given time. This stage is essential because it allows the follicle to reset before entering the resting phase.
Disruptions in the catagen phase, such as stress or hormonal imbalances, can prematurely push more hairs into the next phase, leading to increased shedding.
Telogen Phase: The Resting Stage
The telogen phase is a resting period that lasts about three to four months. During this phase, the hair remains in place, but the follicle remains dormant, undergoing a period of inactivity.
About 10-15% of scalp hairs are in the telogen phase at any given time. While no new hair growth occurs, the follicle is still alive and preparing for the next cycle. Once the telogen phase ends and the old hair is shed, the follicle reactivates, and a new anagen phase begins.
Factors such as stress, illness, or poor diet can cause excessive numbers of hairs to enter the telogen phase at once, leading to a temporary condition called telogen effluvium, which results in noticeable shedding.
Exogen Phase: The Shedding Stage
The exogen phase is an extension of the telogen phase, during which the old hair sheds to make room for new growth. This natural shedding process occurs daily, with most people losing 50 to 100 hairs per day.
Shedding can be influenced by seasonal changes, diet, and overall health. If excessive hair falls out and is not replaced at a normal rate, it can lead to noticeable thinning.
Factors That Influence Hair Growth
Hair growth typically occurs at a rate of half an inch per month but is influenced by a variety of internal and external factors. While genetics help to determine hair length, thickness, and growth rate, lifestyle choices and environmental conditions also affect hair health.
Factors such as nutrition, hormones, stress, and daily hair care practices can either support or disrupt the hair growth cycle.
Understanding these influences can help you take proactive steps to maintain strong, healthy hair and minimize issues such as thinning, breakage, and excessive shedding.
Overall Health
General health has a direct impact on hair growth. The body prioritizes vital functions, so when health is compromised, hair growth may slow or stop. Chronic illnesses, autoimmune conditions, and infections can all disrupt the hair cycle.
A healthy scalp is also essential—conditions such as dandruff, psoriasis, or scalp infections can weaken hair follicles, leading to shedding and slow regrowth. Proper hydration, circulation, and oxygenation of the scalp help nourish the follicles and ensure consistent growth.
Genetics and Hormones
Genetics determines hair characteristics, including density, texture, and growth rate. Some people naturally have longer anagen phases, which allows their hair to grow longer.
Hormones also impact hair growth. Androgens such as testosterone and dihydrotestosterone (DHT) can shrink hair follicles in those prone to pattern hair loss. Estrogen, however, supports hair growth, which explains why hair often thickens during pregnancy and sheds postpartum.
Hormonal imbalances, such as thyroid disorders or polycystic ovary syndrome (PCOS), can have significant negative effects on hair health.
Age and Sex
Hair growth changes throughout life. During youth and early adulthood, the anagen phase tends to be longer, allowing for thicker, faster-growing hair. As people age, hair follicles shrink, the growth phase shortens, and shedding may increase, leading to thinning.
Men and women experience different hair growth patterns due to hormonal influences. Men often have higher levels of DHT, which can contribute to male pattern baldness, while women may experience diffuse thinning, especially after menopause, due to declining estrogen levels.
Nutrition
Nutrients provide the building blocks for hair growth. Protein is essential as hair is primarily composed of keratin, a protein structure. Key vitamins and minerals such as biotin, iron, zinc, and vitamins A, C, D, and E support follicle health and cell turnover.
Iron deficiency, in particular, is a common cause of hair shedding. Omega-3 fatty acids from foods such as fish and nuts help maintain scalp health, while a balanced diet rich in whole foods ensures that the follicles receive the necessary nutrients for optimal growth.
Environmental Factors and Daily Practices
External factors such as pollution, UV radiation, changes in seasons, and harsh weather conditions can damage hair and slow growth. Frequent exposure to chemicals, such as chlorine in pools or smoke from air pollution, can weaken the cuticle, making hair brittle.
Daily habits also impact hair health—tight hairstyles, excessive heat styling, and harsh chemical treatments can cause breakage and follicle damage. Gentle hair care routines, including proper cleansing, conditioning, and avoiding excessive manipulation, help protect hair.
Stress
Stress can significantly impact the hair growth cycle by pushing more follicles into the telogen (resting) phase, leading to increased shedding—a condition known as telogen effluvium.
High stress levels trigger hormonal changes, particularly increased cortisol production, which can disrupt follicle activity. Stress-related hair loss is often temporary, but chronic stress may lead to prolonged thinning.
Managing stress through relaxation techniques, regular exercise, and adequate sleep can help support a healthy hair growth cycle.
Smoking
Smoking negatively affects hair growth by reducing blood circulation to the scalp, which deprives hair follicles of oxygen and essential nutrients. The toxins in cigarettes can also damage DNA within follicle cells, accelerating hair thinning and premature graying.
Vaping can contribute to hair loss by reducing blood circulation to hair follicles and exposing the body to harmful chemicals that disrupt hair growth.
Quitting smoking or vaping can improve circulation and scalp health, allowing follicles to function more efficiently and supporting stronger, healthier hair growth.
Common Hair Growth Concerns
Hair growth doesn’t always follow a smooth, predictable pattern. Many people experience changes in growth rate, thickness, and density due to various internal and external factors.
While mild fluctuations are normal, persistent slow growth, thinning, or excessive shedding can be signs of underlying issues.
Slow Growth
Slow hair growth may result from genetics, poor scalp circulation, nutritional deficiencies, frequent heat styling, product buildup on the scalp, or underlying health conditions.
Hair typically grows about half an inch per month, so noticeable sluggishness may indicate a problem. A simple way to check growth is by tracking length changes over several months. Scalp massages, proper hydration, and a nutrient-rich diet can help stimulate growth.
Lack of Growth
When hair appears to stop growing altogether, it may be stuck in the telogen (resting) phase. This can occur due to stress, hormonal imbalances, aging, or damage to hair follicles.
If hair remains the same length despite avoiding trims, weak follicles or an unusually short anagen (growth) phase may be to blame.
Checking for breakage is important—sometimes, hair is growing but breaking before it reaches noticeable length. Addressing scalp health and nutritional deficiencies can encourage regrowth.
Mild Thinning
Thinning hair often starts subtly, with a decrease in density rather than bald patches. It may appear less voluminous, with a wider part or increased scalp visibility. Causes include genetics, hormonal shifts, poor nutrition, or frequent chemical treatments.
Running fingers through the hair or examining the hairline can help detect early thinning. Strengthening hair with protein treatments, reducing heat styling, and ensuring adequate vitamin intake can help maintain thickness and prevent further loss.
Excessive Shedding
Losing 50–100 hairs per day is normal, but excessive shedding can be alarming. If large amounts of hair fall out when brushing, washing, or running hands through the scalp, it may indicate telogen effluvium or another issue.
Stress, postpartum recovery, illness, or nutritional deficiencies can trigger sudden shedding. A simple test involves gently pulling a small section of hair—if multiple strands come out easily, shedding may be excessive.
Identifying and addressing the root cause can help restore normal hair retention. For excessive shedding resulting in noticeable thinning or bald patches, seeing a dermatologist is the best course of action.
Hair Loss Solutions
In addition to improving your diet, practicing gentle hair care, and reducing stress, those experiencing thinning or hair loss have several options for slowing shedding and promoting regrowth.
From topical treatments and oral medications to advanced therapies, a variety of solutions target different causes of hair loss. The right approach depends on factors such as the underlying cause, hair loss severity, and individual response to treatment.
OTC Minoxidil
Minoxidil is an FDA-approved topical treatment available over the counter in 2% and 5% strengths. It works by increasing blood flow to the scalp and extending the anagen (growth) phase of hair follicles.
Applied directly to the scalp, minoxidil is effective for male and female pattern hair loss, helping to slow shedding and promote regrowth. Consistent use is required to maintain results.
Prescription-Strength Minoxidil
Prescription minoxidil products, available in higher concentrations and compounded formulations, offer a more potent alternative for those who do not respond to standard OTC versions.
Dermatologists can prescribe topical formulations or oral minoxidil, which has shown promising results in stimulating hair growth systemically. This option is often recommended for those with extensive thinning or who experience scalp irritation from topical formulas.
Finasteride
Finasteride is a prescription medication that inhibits the enzyme 5-alpha reductase to reduce dihydrotestosterone (DHT) levels.
DHT is a key contributor to male pattern hair loss as it shrinks hair follicles and shortens the growth cycle. By blocking DHT production, products with finasteride help preserve existing hair and encourage regrowth.
Dutasteride
Dutasteride prescription products are a stronger DHT blocker than finasteride, as they inhibit both type I and type II 5-alpha reductase enzymes. It is not FDA-approved for hair loss but is often prescribed off-label for those who do not respond to finasteride.
Spironolactone
Spironolactone is a prescription medication commonly used to treat female pattern hair loss. It works as an androgen blocker, reducing the effects of DHT on hair follicles.
This makes spironolactone products particularly effective for women with hormone-related hair loss, such as that caused by PCOS or menopause. Spironolactone is not typically used for men due to its feminizing effects.
PRP Therapy (Platelet-Rich Plasma)
PRP therapy involves drawing a patient’s blood, processing it to concentrate platelets, and injecting the plasma into the scalp. The growth factors in PRP stimulate hair follicles, encouraging regrowth and strengthening existing hair.
This treatment is often used in combination with other therapies for enhanced results. PRP is a safe, minimally invasive option with minimal downtime, though multiple sessions are usually required to see significant improvement.
Low-Level Laser Therapy (LLLT)
LLLT uses red light or near-infrared laser technology to stimulate hair follicles and improve cellular activity. This noninvasive treatment enhances blood flow to the scalp, prolongs the anagen phase, and may help slow hair thinning.
LLLT is available in the form of laser caps, combs, and in-office treatments. While it can be effective for certain types of hair loss, it is most beneficial when combined with other hair growth treatments.
Final Thoughts on Hair Growth
Hair growth may seem like a simple, automatic process, but beneath the surface, a complex system of cellular activity, hormones, and environmental influences is at work. Knowing how to support and protect your hair can make a lasting difference in its strength and longevity.
Whether you're interested in maintaining healthy strands or seeking products for hair thinning, it all starts with understanding one essential question: how does hair grow? Now that you know the answer, the rest is up to you.
Talk to a board-certified dermatologist to discuss your goals and find the solution that is best for you.
Frequently Asked Questions
How exactly does hair grow?
Hair grows from follicles in the scalp. Cells in the hair bulb divide rapidly, pushing older cells upward, where they harden and form the hair shaft. The process occurs in cycles, with active growth, transition, resting, and shedding phases determining hair length and density.
What causes hair to grow?
Hair growth is driven by a mix of genetics, hormones, and overall health. The hair follicles rely on a healthy blood supply, nutrients, and hormones such as androgens to support cell division. Lifestyle factors, including diet and stress, also influence the rate and quality of hair growth.
Does hair grow from the roots or the ends?
Hair grows from the roots, not the ends. The hair follicle, located in the scalp, produces new cells that push older cells upward, forming the visible hair shaft. Cutting hair does not affect its growth rate, as the follicle’s activity determines how fast and thick hair grows.
What happens every 7 years to your hair?
Hair follows a natural cycle, with individual hairs completing their growth phases at different times. The idea of a "7-year cycle" is a myth, but over time, factors such as aging, hormones, and health changes can alter hair thickness, growth rate, and density.
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