Hair grows from tiny organs in your skin called follicles, and the process is driven by a cluster of specialized cells at the base of each one. On average, a strand of hair grows about half an inch per month, or roughly 6 inches per year. But the real story of hair growth involves a repeating cycle of activity and rest, controlled by chemical signals, hormones, and nutrients that determine how long, how thick, and how fast each strand develops.
How a Single Strand Forms
Every hair on your body begins in a hair follicle, a small tunnel-shaped structure rooted in the skin. At the very bottom of the follicle sits a ball-shaped cluster of cells called the dermal papilla. Think of it as the command center: it sends chemical signals to the cells surrounding it, telling them to multiply. Those surrounding cells, called matrix cells, respond by dividing rapidly, pushing upward, and hardening into the protein fiber you see as a strand of hair.
As matrix cells stack on top of one another and move up through the follicle, they fill with a tough protein called keratin and lose their water content. By the time the strand emerges from your scalp, the visible hair is no longer alive. It’s a tightly packed column of dead, keratinized cells. That’s why cutting hair doesn’t hurt, but plucking it from the root does.
The dermal papilla also relies on growth-stimulating signals to keep this factory running. Two of the key signals it produces encourage the matrix cells to keep dividing. If those signals are disrupted, cell division slows, the follicle shrinks, and hair production stalls.
The Three-Phase Growth Cycle
Hair doesn’t grow continuously. Each follicle cycles through three distinct phases, and every strand on your head is at a different point in that cycle at any given time.
The first phase, anagen, is the active growth period. About 85% of the hairs on your scalp are in this phase right now. On the scalp, anagen typically lasts around two to six years, though the exact duration varies from person to person. The length of your anagen phase is the main factor that determines how long your hair can get before it naturally falls out.
Next comes catagen, a short transitional phase lasting roughly two weeks. During catagen, the follicle shrinks, the lower portion breaks down, and the strand detaches from its blood supply. The hair stops growing but stays loosely anchored in the follicle.
Finally, the follicle enters telogen, a resting phase that lasts about three months on the scalp. The old hair sits in place while a new strand begins forming beneath it. Eventually the old strand sheds, and the follicle re-enters anagen to start the cycle over. Losing between 50 and 150 hairs per day is a normal part of this turnover. At any given time, roughly 10 to 15% of your scalp hairs are in telogen.
What Triggers a New Growth Cycle
The restart of hair growth depends on stem cells that live in a region of the follicle called the bulge, located partway up the follicle’s outer wall. These stem cells remain dormant during the resting phase. When the dermal papilla migrates close enough to the bulge at the end of telogen, it sends activation signals that wake the stem cells up.
Once activated, the stem cells produce rapidly dividing daughter cells that travel downward to rebuild the lower portion of the follicle. This regenerates the hair bulb, restarts matrix cell division, and a new strand begins to form. The entire lower half of the follicle is essentially rebuilt from scratch with each new cycle, which is why hair regrowth is possible even after a strand falls out naturally.
How Hormones Shape Hair Growth
Hormones are among the most powerful regulators of hair growth, and different hormones can either promote or suppress it depending on where the hair is located on the body.
Androgens, the group of hormones that includes testosterone and its more potent derivative DHT, play a dual role. On the face and chest, androgens stimulate thicker, darker hair growth after puberty. On the scalp, however, DHT can bind to receptors in genetically sensitive follicles and gradually shrink them. This miniaturization process is the central mechanism behind pattern hair loss in both men and women. The follicles don’t die. They progressively produce thinner, shorter strands until the hair becomes nearly invisible.
Thyroid hormones also have a direct effect. Research published in The Journal of Clinical Endocrinology & Metabolism found that thyroid hormones prolong the active growth phase by suppressing a key signal that normally terminates it. They also boost matrix cell division and stimulate the production of melanin, the pigment that gives hair its color. This helps explain why people with underactive thyroid conditions often notice thinning, dry hair alongside their other symptoms.
Estrogen generally supports hair growth by extending the anagen phase. During pregnancy, elevated estrogen levels keep more follicles in anagen simultaneously, which is why many women experience noticeably thicker hair. After delivery, estrogen drops sharply, and a large number of follicles shift into telogen at once, causing the temporary shedding many new mothers experience a few months postpartum.
Nutrients Your Follicles Need
Hair is built from protein, so adequate protein intake is a baseline requirement. Beyond that, a handful of specific nutrients play documented roles in keeping follicles functioning properly.
Iron is one of the most important. It helps red blood cells deliver oxygen to the follicle, and low iron stores are one of the most common nutritional contributors to excessive shedding. Premenopausal women need about 18 mg of iron per day, while men and postmenopausal women need about 8 mg. Pairing iron-rich foods with vitamin C significantly improves absorption, and the International Society of Hair Restoration Surgery lists vitamin C alongside iron and vitamin D as the three supplements with the strongest evidence for routine use in supporting hair health.
Vitamin D receptors are present in the hair follicle, and deficiency has been linked to increased shedding. Most experts recommend a daily intake of 2,000 to 5,000 IU for adults, with blood levels checked annually to adjust the dose.
Zinc supports the cell division that drives hair shaft production. The recommended daily intake is 11 mg for men and 8 mg for women. Deficiency can trigger hair loss, but it’s relatively uncommon in people eating a varied diet.
Biotin is perhaps the most marketed hair supplement, though true biotin deficiency is rare. A daily intake of 30 mcg is enough to prevent deficiency. Hair loss specialists sometimes recommend higher doses, but the evidence that supplementing biotin helps people who aren’t deficient is limited.
Why Hair Grows Differently on Different Body Parts
The length your hair can reach on any given body part is determined almost entirely by the duration of the anagen phase in those follicles. Scalp follicles stay in anagen for years, which is why head hair can grow very long. Eyebrow follicles have an anagen phase of only a few months, so the hairs reach a short length and then cycle out. Arm and leg hair follicles have even shorter growth phases.
This is also why shaving doesn’t make hair grow back thicker or faster. Shaving cuts the strand at the surface without affecting the follicle or its programmed growth cycle. The blunt tip of a freshly cut hair can feel coarser as it grows out, but the strand’s actual thickness, color, and growth rate remain unchanged.
Factors That Slow or Disrupt Growth
Stress is one of the most common disruptors. A significant physical or emotional stressor can push a large percentage of follicles into the resting phase prematurely, a condition called telogen effluvium. The shedding usually appears two to three months after the triggering event and resolves on its own once the stressor passes, though regrowth can take six months or longer.
Age gradually reduces hair growth speed and strand diameter. Follicles spend less time in anagen as you get older, and some follicles stop producing visible hair altogether. This is a separate process from hormonal hair loss, though the two often overlap.
Tight hairstyles that pull on the follicle over long periods can cause a form of hair loss called traction alopecia. The constant tension damages the follicle structure, and if maintained for years, the loss can become permanent. Heat styling and chemical treatments don’t affect the follicle itself but can weaken the exposed shaft, leading to breakage that mimics thinning.
One persistent myth worth correcting: poor blood circulation to the scalp does not cause hair loss. Johns Hopkins Medicine lists this explicitly among the misconceptions. While blood flow delivers nutrients to the follicle, the growth cycle is governed by hormonal and genetic signals, not by how vigorously you massage your scalp.