Hair growth is driven by a cluster of specialized cells at the base of each hair follicle called the dermal papilla. These cells act as a command center, sending chemical signals that tell surrounding cells to multiply, stack on top of each other, and push upward to form a hair shaft. Your scalp holds between 90,000 and 150,000 follicles, each cycling independently through phases of growth, rest, and shedding. On average, a strand of hair grows about half an inch per month, or roughly six inches per year.
The Dermal Papilla: Your Hair’s Command Center
Every hair on your body originates from a tiny pocket in the skin called a follicle, and at the bottom of each follicle sits the dermal papilla. This small cluster of cells regulates the growth, shape, size, and even color of the hair it produces. It does this by releasing a variety of signaling molecules that instruct the cells above it to divide rapidly and differentiate into the structures that form a hair strand.
The number of dermal papilla cells in a given follicle directly determines how thick the resulting hair will be. Follicles with more of these cells produce thicker, coarser strands. Follicles with fewer produce fine, wispy ones. This is why the hair on your scalp differs so dramatically from the hair on your forearm. It’s also why hair can become thinner over time if dermal papilla cells are lost or stop functioning properly.
The Four Phases of the Hair Cycle
Each follicle cycles through distinct stages independently. At any given moment, different hairs on your head are in different phases, which is why you don’t shed all your hair at once.
Anagen (growth): This is the active phase, lasting two to eight years for scalp hair. Cells in the hair bulb divide rapidly, building the shaft upward while pigment cells add color. The length of this phase determines how long your hair can get before it stops growing. Eyebrow follicles, by contrast, stay in anagen for only two to three months, which is why eyebrows never grow as long as scalp hair.
Catagen (regression): A brief transitional phase lasting about two weeks. The follicle shrinks, detaches from its blood supply, and cells in the bulb undergo programmed cell death. The hair stops growing but remains anchored in place.
Telogen (rest): The follicle rests for two to three months. About 9% of scalp follicles are in this phase at any time, compared to 40 to 50% of hair on the trunk. While the old hair sits idle, a new hair begins forming at the base of the follicle.
Exogen (shedding): The new hair growing beneath eventually pushes the old strand out. This is the hair you find on your pillow or in the shower drain. It’s a normal, continuous process.
What Triggers a New Growth Cycle
The transition from resting to active growth depends on a carefully timed sequence of molecular signals. Researchers have identified a two-step activation process involving the stem cells that live in a region of the follicle called the bulge. These stem cells are the reservoir that generates each new hair.
During the resting phase, a signaling pathway involving bone morphogenetic proteins (BMPs) keeps stem cells quiet. Growth begins when BMP signaling drops and a competing pathway, driven by a molecule called beta-catenin, ramps up. Beta-catenin enters the cell nucleus and switches on genes that promote cell division. At the same time, the dermal papilla releases growth factors that stimulate the stem cell compartment, effectively doubling the rate at which new cells are produced. These signals work together: no single molecule is enough on its own, but in combination they orchestrate the transition into a new growth phase.
This same beta-catenin pathway is essential not just for cycling but for the initial formation of hair follicles during embryonic development. In animal studies, deleting the gene for beta-catenin in skin cells prevents follicles from forming at all, while artificially boosting it causes extra follicles to develop.
How Hormones Shape Hair Growth
Hormones are among the most powerful regulators of hair growth, and they can either promote or suppress it depending on the body site. Androgens, the group of hormones that includes testosterone, stimulate hair growth on the face, chest, and other body areas after puberty. On the scalp, however, they can have the opposite effect.
A key player in this process is a growth factor called IGF-1, which promotes cell division and tissue remodeling in the follicle. Androgens regulate the production of IGF-1 in dermal papilla cells. In pattern hair loss, the dermal papilla cells from thinning areas of the scalp secrete significantly less IGF-1 than cells from areas that remain full. This helps explain why hair loss follows predictable patterns: the follicles themselves respond differently to the same hormones depending on their location.
Other hormones matter too. Thyroid hormones influence the duration of the growth phase, and estrogen is thought to prolong anagen, which is why many women notice thicker hair during pregnancy when estrogen levels surge, followed by increased shedding after delivery when those levels drop.
Blood Supply and Oxygen Demand
Growing hair is metabolically expensive. During the anagen phase, cells in the hair bulb are among the fastest-dividing cells in the body, and they need a steady supply of oxygen and nutrients to keep up. Blood vessels from deep in the skin surround each active follicle, delivering what it needs and carrying waste away.
Active follicles actually promote the growth of new blood vessels around themselves, a process driven by vascular endothelial growth factor (VEGF). This ensures that the blood supply scales up to meet demand during the growth phase. Loss of blood supply to follicles is associated with some forms of hair loss, though the exact relationship is still being defined. It does appear that anything improving scalp circulation, from massage to certain topical treatments, supports follicle health in part by enhancing nutrient delivery.
Nutrients That Fuel Hair Growth
Because hair cells divide so rapidly, they’re sensitive to nutritional deficiencies. Iron, zinc, and biotin are the nutrients most commonly linked to hair health.
Iron is essential for oxygen transport to follicle cells. Low iron stores are a well-recognized cause of hair shedding, particularly in women. Zinc plays a role in cell division and protein synthesis in the follicle, and deficiency can push follicles into the resting phase prematurely. Biotin, a B vitamin, supports the production of keratin, the structural protein that makes up the hair shaft. Optimal biotin levels are considered to be above 400 nanograms per liter in the blood, while levels below 100 are considered deficient. True biotin deficiency is uncommon in people eating a varied diet, but suboptimal levels are more frequent than many people realize.
Protein intake matters as well, since hair is almost entirely made of keratin. Severe calorie restriction or very low protein diets can shift a disproportionate number of follicles into the resting phase within a few months, leading to noticeable thinning.
Seasonal Changes in Hair Growth
Hair growth isn’t constant throughout the year. A study tracking follicle activity across seasons found that the proportion of scalp follicles in the active growth phase peaked at over 90% in March and fell steadily to its lowest point in September. Shedding peaked around August and September, when about 60 hairs were lost per day, more than double the rate during winter months.
Beard growth follows a similar pattern, with growth rates lowest in January and February and climbing steadily to a July peak roughly 60% above winter levels. Thigh hair showed the same trend, though less dramatically. Interestingly, fingernail and toenail growth showed no seasonal variation at all, suggesting the effect is specific to hair follicles and likely influenced by daylight exposure or UV radiation rather than temperature alone.
Why Hair Differs Across Your Body
The same basic biology drives hair growth everywhere on your body, but differences in follicle programming create enormous variation. Scalp follicles can stay in anagen for up to eight years, allowing hair to grow several feet long. Eyelash follicles cycle through in a matter of weeks. The difference comes down to how long each follicle’s dermal papilla sustains growth signals before triggering the transition to the resting phase.
Hair color is determined during the growth phase by pigment cells called melanocytes, which sit among the dividing cells in the hair bulb and inject pigment into the forming shaft. When these melanocytes stop producing pigment, whether from aging or other factors, the result is gray or white hair. The follicle itself continues to function normally. Hair density also varies by natural color: people with blonde hair average around 150,000 scalp follicles, while those with red hair have closer to 90,000. Brown and black hair fall in between, at roughly 110,000 and 100,000 respectively.