Learn What Science Shows About Hair Growth

Understanding the Hair Growth Cycle

Hair doesn't grow continuously at the same rate. Instead, each strand of hair goes through a natural cycle with distinct phases. Understanding this cycle helps explain why hair falls out, why it grows back, and how long it takes to see changes in your hair.

The hair growth cycle has three main phases. The first phase is called the anagen phase, which is the active growth period. During this time, hair cells divide rapidly at the root, pushing the hair shaft upward. This phase can last anywhere from two to seven years, and the length of this phase partly determines how long your hair can grow. People with longer anagen phases can grow their hair longer than people whose anagen phase is shorter. On average, hair grows about half an inch per month during this active growth phase, though this varies from person to person.

The second phase is called the catagen phase, or transition phase. This is a much shorter period, lasting only about two to three weeks. During this time, hair growth slows down and the hair follicle begins to shrink. The hair is no longer being actively pushed upward by cell division at the root.

The third phase is the telogen phase, or resting phase. This phase lasts about three months. During this time, the hair is no longer growing, but it remains in the follicle. Eventually, the hair falls out, and the cycle begins again with a new hair starting to grow in the same follicle.

At any given time, most of your scalp hair is in the anagen phase—typically about 85 to 90 percent. This is why you don't lose all your hair at once. The remaining hair is distributed among the catagen and telogen phases. Understanding this cycle explains why you shed hair every day, usually between 50 and 100 hairs, and why this is completely normal. It also explains why waiting for results takes time: if you want to grow your hair longer or change your hair care routine, you need to wait for the cycle to complete before you see the full results.

Practical takeaway: Hair growth is a slow process because most hair is actively growing for months or years. If you change your hair care routine, give it at least three to six months to see meaningful changes, since you need to wait for hairs in different cycle phases to respond.

What Genetics and Biology Determine About Your Hair

Your genes have a major influence on your hair. The DNA you inherit from your parents determines many characteristics of your hair, including its color, texture, thickness, and how fast it grows. This is why hair often runs in families—if your parents had thick, fast-growing hair, you're more likely to as well.

Hair texture and type are primarily determined by genetics. Whether your hair is straight, wavy, or curly depends on the shape of your hair follicles. Straight hair grows from round follicles, while curly hair grows from curved or spiral-shaped follicles. The thickness of individual hair strands, called the diameter, is also genetically determined. People with naturally thick hair have larger-diameter strands, while people with fine hair have smaller-diameter strands. Neither type is better or worse—they're simply different.

The rate at which your hair grows is also largely genetic. Research shows that hair growth rates vary significantly from person to person, with growth rates ranging from about 0.3 millimeters per day to over 0.4 millimeters per day. Over a year, this difference means some people's hair grows several inches more than others. Studies have found that hair growth rate can vary based on ethnicity as well. For example, research published in the dermatology literature suggests that people of African descent may have different hair growth rates and patterns compared to other groups, though individual variation within any population is significant.

Age also affects hair growth through biology. Hair grows fastest during your twenties and thirties. As you age, the anagen phase becomes shorter, and hairs may not grow as long or as thick as they did when you were younger. Additionally, the percentage of hair in the resting phase may increase with age, which can lead to more noticeable shedding. By age 50, more than half of people experience some degree of hair thinning due to these natural changes.

Hormones also play a significant biological role in hair growth. Androgens, a group of hormones that includes testosterone, affect hair growth patterns. These hormones can make scalp hair thinner while making body hair thicker in certain areas. This is why some people experience pattern baldness or hair thinning as they age, particularly if they have genetic sensitivity to these hormones. Women may notice changes in hair thickness during pregnancy, menopause, or when taking certain hormonal medications, because these events change hormone levels.

Practical takeaway: You cannot change your genetics, but understanding your inherited hair characteristics helps you set realistic goals. If your parents had fine hair, you likely will too, and that's not something to fight against—instead, you can learn techniques that work specifically for fine hair.

How Nutrition and Health Affect Hair Growth

What you eat and your overall health status directly affect how well your hair grows. Hair is living tissue that requires specific nutrients to grow strong and remain in the growth phase. If your body isn't getting adequate nutrition, one of the first places this shows up is in your hair, which may become thinner, weaker, or shed more noticeably.

Protein is essential for hair growth because hair is made primarily of a protein called keratin. Your body breaks down the protein you eat into amino acids and uses these to build keratin in your hair follicles. Research shows that people who don't get enough protein may experience increased hair shedding and slower hair growth. Good sources of protein include meat, fish, eggs, dairy products, legumes, nuts, and seeds. Most adults need about 0.8 grams of protein per kilogram of body weight daily, though some research suggests that people concerned about hair health may benefit from adequate protein intake.

Iron is another critical nutrient for hair growth. Iron helps red blood cells carry oxygen throughout your body, including to your hair follicles. Without adequate iron, follicles don't get enough oxygen, and hair may enter the resting phase prematurely, leading to shedding. This is why iron deficiency anemia often causes noticeable hair loss. Studies have found that people experiencing unusual hair shedding sometimes have low iron levels. Women are at higher risk for iron deficiency because of menstrual blood loss. Good sources of iron include red meat, poultry, fish, beans, and leafy greens.

Zinc, biotin, and B vitamins also support hair growth. Zinc helps with cell division and protein synthesis in hair follicles. Biotin, a B vitamin, is involved in producing keratin. B vitamins help your body convert food into energy, which follicles need for active growth. Vitamin D, which your body makes when your skin is exposed to sunlight and which is also found in some foods, may play a role in hair follicle cycling. A study published in dermatology research found that people with hair loss sometimes have lower vitamin D levels, though more research is needed to understand this connection fully.

Overall health conditions affect hair growth significantly. Chronic stress, thyroid disorders, autoimmune conditions, and severe infections can all trigger increased shedding or slower growth. When your body is under stress or fighting illness, it redirects resources away from non-essential functions like hair growth. Additionally, certain medications, including some blood pressure medications and chemotherapy drugs, can affect hair growth as a side effect.

Practical takeaway: Eating a balanced diet with adequate protein, iron, and other nutrients supports hair growth. If you're experiencing unusual hair loss or very slow growth, consider whether your nutrition and overall health are adequate, and discuss concerns with your doctor if changes persist.

The Role of Scalp Health in Hair Growth

The scalp is the foundation for healthy hair growth. Your hair follicles are rooted in the scalp, and the condition of your scalp directly affects whether those follicles can produce strong, healthy hair. A healthy scalp has balanced oil production, good blood circulation, and low inflammation.

Scalp oil, called sebum, serves an important purpose. It protects the scalp and hair shaft from damage and keeps them moisturized. However, when oil production is imbalanced—either too much or too little—it can interfere with healthy hair growth. Excessive oil creates an environment where bacteria and fungi can overgrow, leading to conditions like seb