The Science Behind How Hair Grows

Have you ever wondered how and why your hair grows? Why do some people experience fast hair growth and others have trouble gaining any length? Everyone's hair growth is different and unique; it’s what gives each person their distinctive look. This informative guide will explain the science behind hair growth and what's actually going on that causes us to have to get haircuts every month.

Hair is beyond the strands we see coming from the scalp. It has two main components: skin cells and protein. Both the protein, scientifically known as keratin, and the structure that constitutes the hair you see on your body are all dead cells. However, despite the cells being dead, they are actively responsible for the process of hair growth. They usually perform 90% of the work. Therefore, when we talk about hair growth, it is more about what happens beneath the skin and the role each part plays.

Hair Structure

Hair has two distinct structures: the follicle, which we don't see because it is below the scalp, and the shaft, which is what is visible above the scalp.

The Follicle

The hair follicle usually looks like a tunnel and runs down from the epidermis into the dermis. The structure contains different layers, each with its own separate function. The shape of the follicle determines whether your hair grows as straight, wavy, or curly. If your follicles are round, you will have straight hair, if your follicles are oval, you will have wavy hair, if they are flat, you will have curly hair. Let's dive into the follicle's structural details.
At the base of the follicle is the hair papilla. This part contains tiny blood vessels called capillaries that bring nourishment. Surrounding the papilla is the live part of the hair which is commonly called the bulb. The bulb is at the lowest part of the follicle, and it is where all the living cells reside. An amazing fact to note is that the cells here are the fastest multiplying cells in the body. They divide every 23-72 hours!
The follicle has two covering layers to protect the growing hair: the inner and outer sheath. The inner sheath usually grows with the hair until the beginning of the sebaceous gland or what people commonly refer to as the oil gland. The outer sheath continues with the strand to the gland itself and connects to it using the arrector pili muscle. The muscle attaches the gland to the fiber-like outer layer of the sheath.
This is also the muscle responsible for the standing of hair. When it contracts, the hair stands, and the sebaceous gland produces oil for the scalp. This oil gland produces sebum which is essential in conditioning the scalp and hair. It is most active during puberty, but its production decreases as one continues to age. According to scientists, the gland produces more sebum in men than in women, even as they continue to age.

The Hair Shaft

The shaft is generally the solid protein commonly known as keratin. The shaft only has dead cells. It comprises three main layers: the cuticle, medulla, and cortex. The medulla is the innermost layer and contains a thin layer of cells. The next layer is the cortex, which usually makes the more significant portion of the strand. It has long keratin cells, which usually bind together using hydrogen and sulfur. The cortex and medulla are responsible for hair color or pigmentation.
The cuticle usually contains shingle-shaped overlapping scales. It behaves like the top cover for your shaft and minimizes the loss of moisture. However, poor maintenance and weathering can affect this layer and reduce its functionality. The cuticle usually gives the strands a shiny, oily look in healthy hair.

Hair Growth

Hair follicle development begins when a baby is in the womb. By week 22 of the pregnancy, a fetus has about 5 million follicles. These will be the maximum follicles they have all their lives since the body does not make more as a person continues to grow. Of these 5 million, 1 million of them are on the head.
Hair growth usually happens below the surface. The dead cells are just an extension of the things happening beneath the scalp. Midway up the follicle, there is a part called the Adamson's fringe where the shaft gets keratinized and becomes dead cells. This junction is critical and prevents skin infections like ringworms from affecting the structures below it. Therefore, your hair growth continues normally despite what is happening on the skin.
There are three stages of hair growth. Every follicle undergoes all of them to get the increased length of hair we experience on the surface.


This is the stage of active growth of the follicles. It lasts two to seven years, and the stage length varies with individual genetics. Anagen mainly involves the rapid multiplication of the cells in the bulb at the base of the hair papilla. Your hair can grow up to 1cm every 28 days. By the end of the cycle, maximum hair growth ranges between 18 to 30 inches. However, the growth rate and length depend entirely on a person's genes. Some have a short cycle, while others have a long one.
It is also crucial to note that the hair on the head has a longer growth cycle than the hair on other body parts. For the arms, legs, and facial hair, their cycle is mostly between 30-45 days. This short period explains why the hair usually doesn't grow to be very long.


This is a short transition time where your hair stops growing because the shaft detaches from the bulb, thus cutting the blood supply. The phase can last between ten days to three weeks. Since there is no nutrient supply, the shaft will eventually shed off. When it falls off, a new hair shaft attaches itself to the bulb, and this one is called club hair.


The telogen phase is the resting time and lasts around 100 days for the hair on the scalp, but may be longer for the other body hair like eyelashes. The long resting phase reduces the length and growth rate. It ends the growth cycle and involves shedding off mature hair. If you pull out a strand of hair that is at this stage, it will come off with a whitish dry solid at the base, which is the strand's root. However, this is not the case for broken hair. A broken strand will have a jagged end, and it will not have the full length of the strand.
During this phase, the follicle isn't multiplying or growing, and club hair sits in the papilla, waiting for the upcoming growth cycle. Shedding is very healthy and only shows that your hair is ready to produce more hair. When the strand falls off, the follicle will remain at rest for about three months, preparing to start the process with the club hair after a while. In most cases, a healthy head sheds about 25-100 strands a day. Any loss beyond that range signifies unhealthy hair.

Growth Factors

Each hair follicle grows independently and goes through the growth cycle at various times. This variation helps you have a full head of hair. It is why all your hair does not fall out at once. In a regular cycle, about 85% of your hair is at the Anagen stage at each particular time and about 15% at the Telogen stage. Therefore, the hair that you shed should always be less than what is continuously growing.
Apart from the natural process, you can help stimulate your hair growth. How much care you put into your hair maintenance determines how well the strands grow. If you neglect your hair, you may get a shorter cycle or weaker structure.
 To aid the production of healthy long hair, you can do the following:
  • Wash your hair regularly
  • Air dry your hair after washing
  • Keep your body and scalp hydrated
  • Avoid heat styling techniques
  • Oil massage your head
  • Comb gently
  • Avoid chemical hair products
These simple actions can boost your Anagen stage and ensure you get the best from the whole cycle. However, factors like nutritional deficiencies, stress, long-term treatment, and illnesses can cause hair thinning and loss. These issues can cut short your Anagen phase and cause many strands to enter the resting phase simultaneously. With that, you start shedding more strands per day. This process is scientifically known as telogen effluvium or diffuse hair loss. If the process keeps repeating, your hair will rarely have enough time to grow, and thus your follicles will start producing thinner hair.

Length Retention

Your hair length depends on how much or how little your hair breaks off at a time. If your hair breaks off often, then it is often because it is dry or weak. You can have well-moisturized hair by taking in a lot of water and ensuring your hair gets conditioned. It is essential to wash your hair as often as possible and oil it to prevent it from losing moisture, especially during the hot summer weather. You can also try deep conditioning to help your hair get enough nutrients.
If you have weak, brittle hair, then it shows that your protein levels are low. You can improve this by increasing the amount of protein intake in your diet or using hair treatments with protein supplements. However, before you start your protein treatment, visit a hair specialist and learn the amount of protein your follicles can take. If your hair is too sensitive, using protein-saturated products may cause further weakening and breakage. Leave-in conditioners, stylers, and moisturizers can also help you get enough nutrients for your hair.
Having a balance between these two items is crucial. If you have more moisture than protein content, your hair will become inelastic and mushy. On the other hand, too much protein without water makes the hair brittle and stiff. Having the proper balance suitable for your hair will help you enjoy the feel of your hair and keep it strong for the entire Anagen stage.


You can easily know how to take care of your hair and understand it. Each person's genetics play a significant role in the outcome of the hair growth, cycle, and length, but you must supply your follicles with what they need. Whether you have blonde, dark, or red hair, or whether your hair is straight or curly, give it the best nutrients, and you will enjoy healthy long hair with minimum loss.