The short answer: The hair growth cycle determines how long hair grows, but it can get interrupted by reactive oxygen species—highly reactive molecules that damage the hair follicle (where hair comes from). Worsened by oxidative stress, the normal hair growth cycle is disrupted. Hair follicles get pushed out of the active growth phase too early, so hair spends less time growing and sheds faster. To get the hair growth cycle back on track, lower hair stress by strengthening + shielding hair from daily damage. 

Notice your client’s hair hasn’t grown since their last appointment? You’re not alone in hearing client frustrations about why their hair takes so long to grow, why it can’t grow past their shoulders, or why their ponytails are thinner than they used to be. Even after trying hair growth DIYs, from rosemary oil spritzes to castor oil massages, their hair may still shed or stop growing well before their goal. 

It’s a call to look beyond the surface—because the lack of hair growth is not a cosmetic issue, it’s a biological one. Their body is talking in the way it knows how: sending increasingly desperate SOS signals in the form of hair shedding and thinning.

Understanding why hair starts shedding, thinning, or stops growing can get you one step closer to the solution. That’s why we tapped Dr. Rachael Williams, K18 Sr. Scientist and Scalp + Hair Follicle Biology Expert, to break it down for you.

first: how does hair grow?

“Did you know that hair has 100,000—150,000 hair follicles within the scalp? A hair follicle is a mini organ that makes the hair fiber. At the base, the bulb is made of different cells. There’s a group of cells called the dermal papilla that is the master orchestrator of the hair cycle and follicle size,” explains Dr. Williams. 

the hair growth cycle stages

Follicles cycle through an active growth phase, a regression phase, and a shedding phase. Even if hair feels like one unit growing together, each hair follicle actually goes through hair growth cycles on its own—undergoing ten to thirty cycles in a lifetime! Plus, each hair growth cycle can take years to go from one part to the next. This can explain why some areas of your clients’ hair look different or grow longer than others. 

Let’s dive into what happens during each hair cycle phase.

anagen

According to Dr. Williams, “The hair follicle goes through an active growth phase, and this is where pigment is produced in the hair. How long the cycle is determines how long your hair will be, lasting approximately 2-8 years.”

Fun fact: That’s why eyebrow hairs aren’t as long as scalp hair, staying in anagen for only 2-3 months. 

Now the tough news. The anagen phase gets shorter with age, so hair gets weaker and thinner over time. It can also stop earlier than expected, so the hair fiber sheds and ends up in the hair brush. That brings us to the next phase…

catagen

“This is where the fiber detaches from the dermal papilla and the blood supply, and is held loosely within the scalp. It lasts approximately 3 weeks,” explains Dr. Williams. The newly formed hair shaft makes its way up towards the hair follicle bulge. But if it can’t reach the bulge, the hair cycle ends, and that hair is lost. 

telogen

Dr. Williams sheds light on “telogen, which is a maintenance phase. Within this phase, the anchoring proteins, which are holding the fiber in place, start to get broken down.” New hair is developing at the base of the hair follicle, pushing this hair shaft up to the surface. The telogen resting phase lasts about 2-3 months. 

exogen

This is the last, and shortest cycle, overlapping with the next anagen phase. Dr. Williams shares that “once the proteins are broken down, the hair sheds. It’s making way for the already forming hair fiber below in the next hair cycle.”

Your clients typically shed about 100-150 hairs a day. It may seem like a lot, but most have about 100,000 scalp hairs at any time: so don’t fret if you see some hair fall during the appointment. 

why does your hair stop growing?

Here’s where the culprit comes in: premature hair aging from reactive oxygen species (ROS). They are highly reactive molecules that interact with surrounding lipids and proteins in the hair + scalp, causing a whole lot of damage. They’re a natural byproduct of cellular processes, but too much can throw a wrench in the hair growth cycle. 

Lifestyle choices can speed up the chaos and cause oxidative stress. Think: poor diet, smoking, and lack of sleep. Even things out of our control, like emotional stress and hormonal changes! Oxidative stress causes inflammation and collagen breakdown that messes with the hair follicles. Dr. Williams emphasizes that, “this oxidative stress overpowers scalp + hair follicles’ natural antioxidant defenses,” so they can’t work properly.

This is when the growth cycle goes off the rails. Hair follicles get pushed out of their active growth phase too early and into catagen (the regression phase) and then telogen (the resting phase). So the hair spends less time growing, and sheds faster than you expect.