Hair grows from tiny organs in your skin called follicles, each one cycling independently through years of active growth, a brief wind-down, a resting period, and eventual shedding. Scalp hair grows about half an inch (1.25 cm) per month, or roughly 6 inches per year, though genetics, age, hormones, and nutrition all shift that number. Understanding what’s happening beneath the surface explains why hair behaves the way it does, from why it stops at a certain length to why it falls out and comes back.
What’s Inside a Hair Follicle
A hair follicle is a small, tube-shaped structure embedded in your skin. The visible strand you think of as “hair” is made almost entirely of keratinocytes, cells that produce a tough protein called keratin. These cells form the hair shaft itself along with protective sheaths that wrap around it like concentric layers.
At the base of each follicle sits a cluster of specialized cells called the dermal papilla. This tiny structure acts as a command center, directing the cells above it to divide, multiply, and organize into a hair strand. The dermal papilla determines the size, shape, and color of each hair, and it controls how often the follicle regenerates. Progenitor cells sitting right next to the dermal papilla generate new cells that divide a few more times, then differentiate into the distinct layers of the hair shaft: the outer cuticle, the structural cortex, and in thicker hairs, a soft inner core called the medulla.
The follicle itself has no blood vessels inside it. Instead, a network of tiny blood vessels surrounds it from the outside, delivering oxygen and nutrients through the skin. During active growth, these surrounding vessels expand dramatically, with research showing a more than fourfold increase in vessel size around follicles during the growth phase. The follicle signals for this extra blood supply by releasing a growth factor that stimulates new vessel formation. When that signal is blocked experimentally, hair growth slows and the resulting hairs are smaller.
The Four Phases of the Growth Cycle
Every hair on your body cycles through four distinct phases, and each follicle runs on its own clock. That’s why you don’t shed all your hair at once.
Anagen: Active Growth
This is when the follicle is producing a hair strand. On your scalp, anagen lasts two to eight years, which is why scalp hair can grow so long. In other areas, anagen is much shorter. Eyebrow follicles, for example, stay in this phase for only two to three months, which is why your eyebrows never reach your chin. The length of the anagen phase is the single biggest factor determining how long any given hair can get. If your follicles tend toward the longer end of that two-to-eight-year range, your hair has the potential to grow longer before it naturally stops.
Catagen: Transition
After growth winds down, the follicle enters a roughly two-week transition. The lower portion of the follicle shrinks and detaches from the dermal papilla. Cells in the bulb of the follicle die off in a controlled process, and the dermal papilla migrates upward toward a region called the bulge, where stem cells are stored.
Telogen: Rest
The follicle then enters a resting phase lasting about two to three months on the scalp. At any given time, roughly 9% of your scalp hairs are in this phase. On the trunk of your body, that number jumps to 40 to 50%, which partly explains why body hair stays short. The old hair sits in the follicle, anchored loosely, while beneath it a new hair quietly begins to form.
Exogen: Shedding
The final phase is when the old resting hair actually falls out. The newly forming hair pushes upward from below, displacing the old strand. This is the hair you find on your pillow or in the shower drain. Losing 50 to 100 hairs a day from your scalp is a normal part of this process.
How a New Growth Cycle Starts
The transition from rest back to active growth isn’t random. It’s triggered by a carefully timed molecular conversation between stem cells in the follicle and the dermal papilla below. The follicle’s bulge region houses stem cells that can regenerate the entire lower portion of the follicle, but these cells spend most of their time in a quiet, inactive state.
Two signaling systems control the wake-up call. One keeps stem cells dormant by actively suppressing them. The other activates them. During the resting phase, the dermis (the deeper layer of your skin) releases waves of suppressive signals that prevent the follicle from restarting. These waves eventually subside, creating a window of opportunity. When that suppressive signal drops, the dermal papilla begins releasing activating signals, including growth factors and molecules that specifically stimulate the cells closest to it. These nearby cells respond first, beginning to divide and pulling the follicle back into growth. The bulge stem cells follow shortly after, resupplying the dividing cell population that builds the new hair shaft. This two-step activation ensures the process is orderly rather than chaotic.
What Determines Your Maximum Hair Length
Your hair doesn’t grow forever and then fall out randomly. Each follicle has a genetically influenced maximum time it can spend in the anagen phase, and that duration sets a ceiling on length. At half an inch per month, a follicle with a two-year anagen phase can produce a strand about 12 inches long. One with an eight-year phase could theoretically reach four feet.
For a long time, scientists assumed anagen ended passively when the rapidly dividing cells at the base of the follicle simply ran out of steam. More recent research suggests that anagen termination is actually an actively controlled process. The follicle has a molecular checkpoint that triggers the transition to catagen, and a specific signaling protein acts as the brake. In rare cases of familial trichomegaly, where people grow unusually long eyelashes and body hair, the gene for this braking signal is nonfunctional, allowing anagen to continue far longer than normal. Conversely, people with short anagen syndrome have mutations that cause hair to exit the growth phase prematurely, resulting in hair that never grows past a few inches.
Human scalp hair is unusual in the animal kingdom for its extraordinarily long anagen duration. Researchers believe this evolved through changes in regulatory DNA that suppress the molecular brake on anagen exit, allowing scalp follicles to grow continuously for years while body hair follicles retain much shorter cycles.
How Hormones Change the Growth Cycle
Hormones have a powerful and location-dependent effect on hair. Androgens, the group of hormones that includes testosterone and its more potent derivative DHT, stimulate hair growth on the face, chest, underarms, and pubic area by converting fine, pale vellus hairs into thicker, darker terminal hairs. This is why body and facial hair increase during puberty.
On the scalp, androgens do the opposite. DHT shortens the anagen phase and causes follicles to gradually miniaturize, producing thinner, shorter hairs with each cycle until some follicles stop producing visible hair altogether. This process drives the most common form of hair loss in both men and women.
Estrogen, by contrast, tends to prolong the growth phase on the scalp. During pregnancy, when estrogen levels surge, many women notice their hair becoming thicker and fuller. The high estrogen delays the shedding phase, so fewer hairs fall out. After delivery, when estrogen drops, those retained hairs enter telogen together, causing a noticeable wave of shedding a few months postpartum. A similar pattern occurs after menopause, when declining estrogen leads to a shorter anagen phase and finer scalp hair.
Nutrients That Support Hair Production
Hair is built from protein, so adequate protein intake matters. The amino acid methionine is a key building block of keratin, and cysteine, another amino acid, contributes the sulfur bonds that give hair its structural strength. Taurine also plays a supporting role.
Iron is essential because hair follicle cells are among the fastest-dividing cells in the body, and iron supports the oxygen delivery and cell division those follicles demand. Selenium contributes to the antioxidant defenses that protect follicle cells during their intense metabolic activity. Deficiencies in any of these nutrients can shift follicles prematurely from growth into rest, leading to increased shedding and thinner hair overall.
Because the follicle itself lacks blood vessels and depends entirely on the surrounding vascular network for nutrition, anything that impairs circulation to the scalp, whether poor cardiovascular health, chronic stress, or smoking, can limit the raw materials available to actively growing follicles.
Why Scalp Hair and Body Hair Behave Differently
The difference between scalp hair and body hair comes down almost entirely to how long each follicle stays in anagen. Scalp follicles can remain active for up to eight years. Eyebrow follicles cycle through in two to three months. Arm and leg hair follicles fall somewhere in between, with relatively short growth phases and long resting phases. At any given time, 40 to 50% of body hair on the trunk is in the resting phase, compared to about 9% on the scalp. This means body hair spends proportionally much more of its life sitting still, which is why it reaches only a fraction of the length scalp hair achieves even though the rate of growth per day is broadly similar across body sites.
The telogen duration also varies by location. Eyelash follicles may rest for just a few weeks before cycling back, while scalp follicles can remain in telogen for close to a year. These differences are genetically programmed into each follicle based on its location, shaped by the local signaling environment and hormone receptor density in that part of the skin.