The Hair Growth Cycle, Simplified

The Hair Growth Cycle, Simplified

We all know that taking care of our hair is important. Whether we’re dealing with hair loss, looking to develop a daily care routine, or just trying to support a new style, it’s essential to have an understanding of how hair grows and functions as part of its natural hair growth cycle. Knowing the basics behind the stages of hair growth can be the key to finding effective solutions that are easy to stick with.

Let’s get to the root of it. 

The Hair Follicle

Hair growth begins within the hair follicle, a pocket-like organ found inside the dermal layer of the skin. Over five million hair follicles blanket the human body, with nearly 100,000 residing on the scalp. The number of follicles can vary by natural hair color, while the density can vary by ethnicity.

Let’s break down the key components of the hair follicle itself.

  • Dermal papilla cells are the core component of every hair follicle. They’re surrounded by blood vessels called capillaries, and the division of dermal papilla cells plays a part in how fast your hair grows, what stage of the hair growth cycle your hair follicle is in, and more. It’s also the target for the active ingredient in Revela’s Hair Revival Serum, ProCelinyl™.

  • The hair matrix sits above the dermal papilla, and new hair growth comes from hair matrix cells as they divide and push upward through the skin. They also transfer color to the hair, known as pigmentation, by transferring melanin, the same compound that makes a mole on our skin look darker or gives us a tan when we’re out in the sun.

  • The hair bulb is the larger, round-shaped structure that includes the dermal papilla, hair matrix, and the base of the growing hair shaft. It protects its contents and sits below the surface of the skin.

  • The sheath covers the outside of the growing hair shaft, giving it shape and structure. It includes both the inner root sheath and the outer root sheath.

  • The sebaceous gland, closer to the skin’s surface, produces oil that lubricates our hair as it grows out. Sebaceous glands are everywhere, too—We’re all familiar with clogged sebaceous glands that lead to unwanted pimples and acne!

  • The arrector pili is a tiny muscle that contracts, or tightens, to make our hair stand up. Think goosebumps!

The Hair Shaft

What we think of as a piece or strand of hair is called the hair shaft, and it contains several types of cells in three distinct layers.

  • The inner layer, or the medulla, is the core component of the hair shaft. It’s primarily made up of keratin, a protein that keeps hair strong and gives it structure throughout the hair growth cycle.

  • The middle layer, or the cortex, surrounds the medulla as a layer of protection and support. Also primarily made of keratin, the cortex makes up most of the hair shaft itself.

  • The outer layer, or the cuticle, wraps around the cortex and is the outermost part that we actually touch. Think of the cuticle like protective bark on a tree: The abundance of keratin in the cuticle forms a rougher, scale-like surface that shields the more fragile inner layers.

Believe it or not, the luscious locks of hair that we all run our hands through are actually dead cells! As new cells divide from within the hair matrix during the growth phase of the hair growth cycle, the previous generation of cells are pushed upward, eventually protruding above the epidermal layer of the skin.

Further still, the follicular foundation of your hair helps to determine the type of hair you have and the shape that it takes as it grows out. Circular follicles tend to produce straighter hair, while oval follicles often produce curlier hair.

The Phases Of Hair Growth

As our hair grows, it passes through three distinct phases. Each follicle will spend the majority of its time in the growth phase, but as we age or experience bodily changes, the length of each phase can change.

Anagen Phase

Lasting between 2-7 years for each follicle, roughly 85-90% of the hairs on our head are in the anagen phase at any time. During the anagen phase, our matrix cells are hard at work, dividing nearly every 18 hours to push the hair strand up, up, and away through the skin. While in this phase, our hair grows roughly half a millimeter each day.

Catagen Phase

In contrast to the anagen phase, the catagen phase takes up a literal hair of the hair’s lifespan, lasting 2-3 weeks on average. Because the phase is much shorter, fewer of our hairs are normally in the catagen phase at any given time. 

While hair growth still occurs during the catagen phase, it begins to slow down: By the end of this phase, hair stops growing altogether, becoming what’s known as club hair. Catagen phase represents the transition to the following period of rest, known as…

Telogen Phase

The telogen phase represents the part of the growth cycle when your hair rests and resets. Believe it or not, 10-15% of your hair follicles sit dormant in the telogen phase for 2-4 months at a time! Once this brief hibernation is complete, the follicle can cycle back to the anagen phase, push out the old hair shaft, and begin anew.

Why Do I Still Have Hair?

Taking in all this information at once might give you a few common questions, so let’s answer the obvious burning question on your mind:

If my hair cycles through a resting phase, shouldn’t there be a period where all my hair falls out at once?

Fortunately, no! Because each of our hair follicles start moving through the cycles at different times, our hair growth is staggered. When we lose 50-100 hairs each day, this is totally normal and expected—most of those hairs were likely in the telogen phase already, and were primed to be replaced by new growth as the follicle proceeds to the anagen phase again.

As we’ve discussed in other posts, a variety of factors can influence the length of time our hair follicles spend in each stage of hair growth. Because of the staggered way in which our hair follicles start their growth cycle and move through each stage, changing the length of time in one stage can lead to noticeable differences. Think of it like traffic backing up during rush hour: You’re on a two-lane road and the car in front of you is trying to make a left turn. As more and more cars begin to pile up behind them, the road in front of them begins to open up more since fewer cars can make it past the intersection.

In this case, the most common form of hair loss that results from a disruption in the hair growth cycle is known as telogen effluvium. Telogen effluvium occurs when the hair follicles spend more time in the telogen phase than normal. As a consequence, this means that fewer hair follicles are spent in the anagen phase, or growth stage. When this happens for a prolonged period of time, it becomes known as chronic telogen effluvium.

Using the traffic jam analogy above, think again about the part of the road right past the traffic jam: When the cars get stuck at the intersection, fewer cars are able to move ahead, and the road feels emptier. When more of our follicles are stuck in the telogen phase, not only do more hairs start to fall out at once, but fewer hair follicles are entering the anagen phase to replace them—in this case, unfortunately, it’s our scalp that feels emptier instead!

In Conclusion

When it comes to haircare, there are multiple factors that play key roles in the growth and health of our hair. Our age, sex, hormone levels, and dietary habits all impact how long our hair stays in the anagen phase to grow longer and fuller, as well as when it progresses through the hair growth cycle to the catagen and telogen phases. Most hair products on the market, like shampoos and conditioners, only affect the quality of the hair shaft and can minimize the risk of hair breakage, rather than impacting the growth phases of the hair follicle itself.

By understanding your hair’s basic anatomy, you can learn more about how to help it grow best. Keep these facts in mind to not only maintain the healthiest version of your hair, but to avoid gimmicks and marketing traps for products pushing claims that aren’t scientifically proven!

Profile photo for Evan Zhao

Reviewed by: Evan Zhao, PhD

Evan is a synthetic biologist and chemical engineer. He completed his BS from Caltech, and his MA and PhD from Princeton University. He received the prestigious Schmidt Science Fellowship as a postdoctoral fellow at MIT/Harvard and has published in the world's top scientific journals including Nature, Nature Biotechnology, Nature Chemical Biology, and more.

Written by: Revela Editorial Team

Next Blog


Minoxidil vs. ProCelinyl™: A Tale of Two Serums

When it comes to treating hair loss, unexpected and repurposed ingredients constantly pop up with claims to regrow hair at a fraction of the cost. The reality is that if...

Skin Elasticity: The “Secret” to Tone & Lift

Fine lines and wrinkles are a hot topic across the entire skin care community, especially in anti-aging products. While these two telltale signs become more pronounced as we age, one...

From Nutraceuticals to Drugs, We Are What We Eat

When it comes to managing health issues, whether hair loss or something else entirely, it makes sense to take a look at the fuel we put into our bodies. Are...