The anagen phase is the active growth stage of the hair cycle, when cells at the root of the hair follicle divide rapidly to build new hair. For scalp hair, this phase lasts two to eight years, making it by far the longest of the three stages in the hair cycle. At any given time, more than 90% of the hair on your head is in anagen, which is why most of what you see on your scalp is actively growing.
How Hair Grows During Anagen
Every hair on your body grows from a follicle, a tiny pocket in the skin. At the base of each follicle sits a structure called the dermal papilla, a small cluster of cells that acts as the command center for hair growth. During anagen, the dermal papilla signals the surrounding cells to multiply. Those rapidly dividing cells push upward, hardening into the hair shaft you eventually see above the skin’s surface.
The size of the dermal papilla directly correlates with how thick the resulting hair strand will be. A larger papilla produces a thicker, coarser hair; a smaller one produces a finer strand. This isn’t a fixed feature. The dermal papilla actively expands and contracts over the course of the hair cycle, with cell numbers increasing during early anagen and gradually decreasing as the phase winds down. That’s one reason hair thickness can change over time: the papilla itself is changing.
Why Anagen Length Determines Hair Length
Your hair doesn’t grow forever. It grows for as long as the follicle stays in anagen, then stops. Scalp hair grows roughly half an inch per month, so a person whose anagen phase lasts two years can grow hair to about 12 inches before the follicle shuts down. Someone with an anagen phase lasting six or seven years could grow hair past their waist. This is why some people struggle to grow their hair past a certain length no matter what they do: their genetics set a shorter anagen window.
This same principle explains why eyebrows and arm hair stay short. Eyebrow follicles remain in anagen for only two to three months. Arm and leg hair have similarly brief growth windows of roughly 30 to 45 days. The hair simply doesn’t have enough time to get long before the follicle transitions to a resting phase and the strand eventually falls out.
What Controls How Long Anagen Lasts
Genetics is the primary factor. Your DNA programs each follicle with a rough anagen duration, which is why hair length potential runs in families. But several other forces can shorten the growth phase by pushing follicles out of anagen prematurely.
Hormones play a significant role. Androgens (the group of hormones that includes testosterone) can shrink follicles on the scalp over time, progressively shortening the anagen phase with each cycle. This is the mechanism behind pattern hair loss in both men and women: follicles that once sustained years of growth start cycling through anagen in months, producing thinner, shorter hairs until some stop producing visible hair altogether.
Nutritional deficiencies can also cut the growth phase short. Iron is particularly important because hair follicle cells are among the fastest-dividing cells in the body and depend on iron for DNA synthesis. Vitamin D supports the outer layer of the hair follicle during growth. Zinc, biotin, protein, and essential fatty acids all contribute to maintaining healthy cell division within the follicle. Severe protein malnutrition causes visible hair thinning and loss because the follicle simply lacks the building blocks to sustain growth. Even a vitamin E supplement (tocotrienols) showed measurable increases in hair count in a small trial of 21 volunteers compared to a placebo group.
Age, stress, and chronic illness can also promote an early transition out of anagen into the resting and shedding phases.
What Happens After Anagen Ends
When the growth signal fades, the follicle enters catagen, a brief transitional phase lasting about two to three weeks. During catagen, cell division stops, the lower portion of the follicle shrinks, and the hair strand detaches from its blood supply. The strand itself doesn’t fall out yet; it just stops growing.
Next comes telogen, the resting phase, which lasts around two to four months for scalp hair. The old hair sits loosely in the follicle while a new hair begins forming beneath it. Eventually the old strand sheds (this is the hair you find on your pillow or in the shower drain), and the follicle re-enters anagen to start the cycle again. Because each follicle operates on its own independent timeline, you’re constantly losing and regrowing hairs without noticeable thinning.
Anagen Effluvium: When Growth Is Disrupted
Sometimes the anagen phase is interrupted abruptly, causing widespread hair loss called anagen effluvium. The most common cause is chemotherapy. Chemotherapy drugs target rapidly dividing cells throughout the body, and because hair follicle cells divide so quickly during anagen, they’re especially vulnerable. The toxic insult causes the hair shaft to fracture at or near the scalp, resulting in sudden, diffuse shedding that can begin within days to weeks of treatment.
Chemotherapy isn’t the only trigger. Radiation exposure, heavy metal poisoning, severe protein deficiency, and certain autoimmune conditions like alopecia areata can all damage follicles during the growth phase. The hallmark sign is a tapered fracture point on shed hairs, which distinguishes anagen effluvium from other types of hair loss where the strand falls out intact.
There is also a separate inherited condition called loose anagen syndrome, in which growing hairs are poorly anchored in the follicle. People with this condition, most often young children, can painlessly pull hairs from their scalp because the strands never firmly attach during the growth phase.
Because anagen effluvium targets growing follicles rather than destroying them permanently, hair typically regrows once the underlying cause is removed. After chemotherapy ends, most people see new growth within a few months as follicles re-enter anagen on their own.