Ever wonder why your hair seems to grow at different rates throughout the year, or why you might shed more hair during certain seasons? The answer lies in the fascinating biological process known as the hair growth cycle. This natural rhythm governs how our hair grows, rests, and eventually falls out only to be replaced by new strands in a continuous cycle that spans our entire lives.
Hair doesn’t simply grow continuously until we cut it. Instead, each strand follows a predictable pattern of growth, transition, rest, and shedding. Understanding these cycles can help explain everything from normal hair loss to the effectiveness of hair removal treatments.
The Three Main Phases of Hair Growth
Your scalp contains approximately 100,000 hair follicles, and at any given moment, each follicle is in its own stage of the growth cycle. This staggered timing is actually a good thing imagine if all your hair decided to enter the shedding phase simultaneously!
The hair growth cycle consists of three primary phases: anagen (growth), catagen (transition), and telogen (resting). Some researchers also identify a fourth phase called exogen, which represents the actual shedding of the hair.
During the anagen phase, cells in the root of the hair divide rapidly, adding to the hair shaft. Blood vessels in your scalp feed the hair root, allowing it to grow. On your scalp, this active growth phase lasts between 2-7 years, which explains why scalp hair can grow so long compared to eyebrow or arm hair, which have much shorter anagen phases. Interestingly, the length of your anagen phase is largely determined by genetics, which explains why some people can grow waist-length hair while others struggle to grow hair past their shoulders.
About 85-90% of the hairs on your head are in the anagen phase at any given time. The actual length of this phase varies dramatically depending on where the hair is located on your body. Eyebrow hairs, for example, have an anagen phase of just 30-45 days, which is why they don’t grow very long even if you never trim them.
When the anagen phase ends, the hair enters the catagen phase, a short transitional period lasting about 2-3 weeks. During this time, the hair follicle shrinks and detaches from the dermal papilla (the structure that supplies nutrients to the hair). The hair stops growing but doesn’t fall out immediately.
Finally, the hair enters the telogen phase, a resting period lasting about 3 months for scalp hair. The follicle remains dormant, and while the hair doesn’t grow during this time, it stays attached to the follicle. About 10-15% of your scalp hairs are in this phase at any time. At the end of the telogen phase, the follicle begins to grow a new hair, which pushes the old telogen hair out, resulting in what we perceive as hair shedding.
This cycle explains why it’s normal to lose 50-100 hairs per day. These aren’t hairs that have “fallen out” due to damage they’ve simply completed their full growth cycle and are making way for new growth.
Factors That Influence Hair Growth Cycles
The hair growth cycle isn’t fixed it responds to a variety of internal and external factors. This adaptability explains why your hair might behave differently during different periods of your life.
Hormones play a massive role in regulating hair growth. During pregnancy, increased estrogen levels extend the anagen phase, resulting in thicker, fuller hair. After childbirth, when hormone levels drop, many women experience increased shedding as numerous hairs simultaneously enter the telogen phase a condition called telogen effluvium. This isn’t permanent hair loss but rather a temporary disruption of the normal staggered timing of the hair growth cycle.
Age also affects hair growth cycles. As we get older, the anagen phase typically shortens, while the telogen phase lengthens. This results in finer, shorter hair and explains why older adults often can’t grow hair as long or as thick as they once did.
Nutrition plays a crucial role too. Hair follicles are among the most metabolically active cells in the body, requiring significant nutrients to support growth. Deficiencies in protein, iron, zinc, biotin, and vitamins D and B12 can disrupt normal cycling and lead to increased shedding or slower growth.
Seasonal changes can affect hair growth as well. Some research suggests that humans, like many mammals, may have evolved to shed more hair in late fall, possibly as a biological response dating back to when humans needed thicker hair for winter warmth. A study published in the British Journal of Dermatology found that human hair loss peaked during the fall months, with the telogen phase most common in July and August.
Stress both physical and emotional can push hair follicles into the telogen phase prematurely. During periods of intense stress, the body diverts resources away from “non-essential” functions like hair growth to support more critical survival systems. This can result in noticeable hair shedding 2-3 months after a stressful event.
I once experienced this firsthand after a particularly stressful period at work. About two months after a major project deadline, I noticed significantly more hair in my shower drain. My doctor explained it was likely telogen effluvium triggered by the stress I’d experienced. Sure enough, the shedding subsided a few months later once the affected hairs had cycled out and new growth had begun.
The Science of Hair Growth Disorders
Understanding the hair growth cycle helps explain various hair disorders. Male and female pattern baldness (androgenetic alopecia), for instance, occurs when hair follicles become increasingly sensitive to dihydrotestosterone (DHT), a hormone derived from testosterone. This sensitivity causes follicles to shrink over time, resulting in progressively shorter anagen phases. Eventually, the anagen phase becomes so brief that the hair doesn’t have time to grow beyond very short, fine vellus hairs.
Alopecia areata, an autoimmune condition causing patchy hair loss, results from the immune system attacking the hair follicles, forcing them into a premature telogen phase. The good news is that the follicles typically remain alive beneath the skin, which is why regrowth is possible when the immune response subsides.
Trichotillomania, a condition characterized by the compulsive pulling of one’s hair, can damage follicles if done repeatedly over long periods. However, if the pulling doesn’t damage the follicle itself, the hair will typically regrow once the behavior stops.
Many hair loss treatments work by manipulating the hair growth cycle. Minoxidil (the active ingredient in Rogaine) is thought to extend the anagen phase and increase blood flow to the follicles. Finasteride (Propecia) works by blocking the conversion of testosterone to DHT, thereby reducing the hormone that shortens the anagen phase in pattern baldness.
Low-level laser therapy, another hair loss treatment, appears to stimulate epidermal stem cells in the hair follicle, encouraging them to enter the anagen phase and begin producing hair. This explains why consistency is key with these treatments they work by gradually shifting more follicles into the growth phase.
Hair removal methods also rely on understanding the growth cycle. Laser hair removal, for example, only works on hairs in the anagen phase because these hairs contain the most melanin (pigment) for the laser to target. This is why multiple sessions spaced weeks apart are necessary to catch different hairs as they enter the anagen phase.
The science of hair growth cycles continues to advance. Researchers are exploring ways to manipulate stem cells to activate dormant follicles, developing new compounds that can extend the anagen phase, and investigating how the microbiome of the scalp might influence hair cycling.
Our hair’s growth patterns reflect a complex interplay of genetics, hormones, nutrition, and environmental factors. By understanding these cycles, we can better address hair concerns, whether they involve unwanted loss or unwanted growth. Perhaps most importantly, this knowledge helps us distinguish between normal shedding and problematic hair loss, potentially saving us unnecessary worry about those strands in the shower drain that are simply making way for new growth.
