Every hair on your head grows from a tiny organ called a follicle, and each follicle cycles independently through phases of growth, rest, and shedding over the course of several years. Your scalp holds between 90,000 and 150,000 of these follicles, and at any given moment they’re all at different points in the cycle. That’s why you shed some hair every day while the vast majority keeps growing.
The Four Phases of the Hair Cycle
Hair doesn’t grow continuously. Each follicle moves through a repeating loop of four distinct phases, and the timing of each phase determines how long your hair can get, how thick it looks, and how much you shed.
Anagen (growth): This is the active phase, lasting 2 to 8 years for scalp hair. The follicle pushes out a hair shaft that grows roughly 1 centimeter per month, or about half an inch. Thicker strands tend to grow faster, around 11.4 mm per month, while finer hairs grow closer to 7.6 mm per month. The length of your anagen phase is largely genetic, which is why some people can grow hair to their waist while others plateau at shoulder length. About 85 to 90 percent of your scalp hairs are in anagen at any given time.
Catagen (transition): When a follicle’s growth window closes, it enters a brief transition lasting about 2 weeks. The follicle shrinks, growth slows dramatically, and the hair separates from the blood supply at the base of the follicle. The strand stays attached but is no longer actively lengthening.
Telogen (rest): For the next 2 to 3 months, the hair sits in place while the follicle is essentially dormant. No growth happens, but the hair doesn’t fall out yet either. Meanwhile, deep inside the follicle, the early stages of a new hair begin forming.
Exogen (shedding): Over a period of several months, the old hair loosens and eventually falls out, often during washing or brushing. A new hair is already growing beneath it. Losing 50 to 100 hairs a day during this phase is completely normal.
What Happens Inside the Follicle
A hair follicle is more complex than it looks from the surface. At the very bottom sits a cluster of specialized cells called the dermal papilla, which acts as the follicle’s command center. These cells send chemical signals upward that tell the surrounding tissue when to start producing a new hair. Without a functioning dermal papilla, the follicle can’t generate growth.
Higher up in the follicle, in a region called the bulge, live stem cells that make the whole cycle possible. These stem cells are self-renewing and multipotent, meaning they can produce all the different cell types needed to build a complete hair shaft. When the dermal papilla sends its “go” signal at the start of anagen, the stem cells wake up, divide, and send daughter cells downward to the base of the follicle. Those daughter cells multiply rapidly to form the matrix, the factory floor where the hair strand is actually assembled. The presence and health of these stem cells is directly tied to hair growth. When they’re damaged or depleted, hair loss follows.
The hair shaft itself is made mostly of keratin, a tough structural protein. As matrix cells divide and get pushed upward, they harden, die, and stack together to form the visible strand. The hair you see and touch is technically dead tissue, which is why cutting it doesn’t hurt.
Why Hair Grows Differently Across Your Body
The same basic cycle governs hair everywhere on your body, but the anagen phase varies dramatically by location. Scalp hair can grow for up to 8 years, which is why it reaches significant length. Eyebrow hairs have an anagen phase of only about 4 months, so they stay short regardless of whether you trim them. Arm and leg hair cycles are similarly brief. The maximum length any hair can reach is simply a product of how fast it grows multiplied by how long its anagen phase lasts.
Natural hair color also correlates with follicle count. People with blonde hair tend to have the most follicles, around 150,000, while those with brown hair average about 110,000, black hair around 100,000, and red hair roughly 90,000. Higher follicle counts generally come with finer individual strands.
Hormones and Hair Growth
Hormones are one of the most powerful influences on your hair cycle. The hormone that gets the most attention is DHT (dihydrotestosterone), a byproduct of testosterone. When DHT binds to receptors on certain scalp follicles, it shortens the anagen phase. Over time, each growth cycle produces a thinner, shorter hair until the follicle eventually stops producing visible hair altogether. This is the primary mechanism behind pattern hair loss in both men and women, and it explains why hair thins gradually rather than falling out all at once.
Estrogen has the opposite effect, prolonging the growth phase. This is why many women notice their hair feels thicker during pregnancy, when estrogen levels surge, and then experience a wave of shedding a few months after delivery when those levels drop and a large number of follicles shift into telogen simultaneously.
Thyroid hormones, cortisol, and insulin also play roles. An underactive thyroid can push more follicles into the resting phase prematurely, while chronic stress elevates cortisol in ways that disrupt the normal cycle. These hormonal influences explain why hair changes often accompany broader health shifts.
Nutrition and Hair Growth
Hair follicles are among the fastest-dividing cells in your body, so they’re sensitive to nutritional shortfalls. Iron is the nutrient with the strongest evidence linking deficiency to hair loss. Ferritin, the protein that stores iron in your blood, offers a useful benchmark: levels below 30 ng/mL are highly likely to contribute to hair shedding, levels between 40 and 70 ng/mL represent the minimum range for healthy hair, and levels above 70 ng/mL are considered optimal for growth.
Zinc and biotin appear in many hair supplements, but medical reviews consistently find insufficient evidence to recommend them unless you have a documented deficiency, which is uncommon in people eating a varied diet. In other words, if your iron and overall nutrition are adequate, adding more supplements on top is unlikely to speed up growth or change hair quality. Severe calorie restriction, crash dieting, and protein deficiency can all trigger a noticeable shedding episode a few months later, as the body diverts resources away from non-essential functions like hair production.
Seasonal and Environmental Factors
If you notice more hair in your brush during late summer or early fall, you’re not imagining it. Seasonal hair loss appears to be a real phenomenon. One explanation is that hormonal fluctuations and changes in light exposure during certain months cause a spike in the number of follicles entering the telogen (resting) phase. A few months later, all those resting hairs shed roughly in sync. The effect is temporary and doesn’t indicate a problem, but it can be alarming if you’re not expecting it.
Blood flow to the scalp, UV exposure, and even ambient temperature may subtly influence growth speed, though these effects are small compared to genetics and hormones. The follicle’s internal programming, driven by its stem cells and dermal papilla signaling, remains the dominant force controlling when and how your hair grows.