Each year, scalp hair grows approximately six inches, or three to four millimeters per day. Human hair development and shedding, in contrast to that of other mammals, is unpredictable and does not follow a seasonal or cyclical pattern.3 Human hair grows in a cyclic pattern of growth and rest known as the “hair growth cycle.”
Hair growth is divided into four separate phases, which are as follows: Hair growth phases have been extensively researched to better understand hair growth and prevent or treat premature hair loss.2
The first three phases, anagen, catagen, and telogen, involve hair growth and maturation, as well as hair follicle activity. During the exogen phase, “old” hair sheds, but usually a new hair is ready to replace it.2
After the cycle is finished, a new strand of hair begins to grow. Hair growth rate depends on age, genetic predisposition, and several environmental factors.7
1. Anagen: Growing Phase
The anagen phase is the first in hair growth. On average, a single hair grows for 3 to 5 years, however for certain people, it might grow for 7 or more years.2
Hair root cells are rapidly dividing. A new hair grows, pushing the club hair (a hair that has stopped growing or is no longer in the anagen phase) up the follicle and out. It takes around 28 days for the hair to develop 1 centimetre at this period.
To nourish the strand, the follicle burrows itself into the dermal layer of the skin, dividing cells surrounding the papilla to make new hair fibers. During this period, the hair bulb makes melanin.1 Approximately 85%–90% of one's hairs are anagen at any given time.7
The anagen process is split into proanagen and metanagen. Proanagen recognises the follicle hair progenitor cells multiplying and starts the differentiation process. The metanagen phase is marked by the appearance of a new hair shaft on the skin surface. The anagen phase can last years.8
The length of this phase is determined by heredity, age, health, and other variables. 5
2. Catagen: Transition Phase
After the Anagen Phase, your hair cycle enters a brief transitional phase known as the Catagen Phase, which typically lasts 10 days or less. Hair follicles shrink and hair growth slows down during this chapter, signalling the end of active hair growth and cutting off individual hairs from the blood supply and cells that generate new hair 4. During the final stages of hair growth, the hair also separates from the hair follicle's base, but it remains in place. Only around 5% of the hairs on your head are in the catagen phase at any given time.2
This is also the moment when the development of a club hair, an important prognostic indication in assessing hair pathology, develops. It is possible to appear thinner if numerous hairs develop club hair at the same time and are then sloughed off. Hypothyroidism, hyperthyroidism, stress, vitamin deficiency, and after childbirth are a few of the factors that can cause this to develop. 8
An epithelial cell column pushes the hair bulb to the surface as it keratinizes (takes on the appearance of a "club hair"). There's a characteristically corrugated, long column of cells at this point, but it'll eventually shorten gradually from the bottom up. The secondary follicular germ is a tiny, nipple-like form of the primary follicular germ. The dermal papilla rises together with the epithelial sac at this stage. 1
3. Telogen: Resting Phase
The Telogen Phase is the third stage of your natural hair development cycle. Strands stay in their follicles but are not actively growing.4 The telogen phase lasts 3 months. This phase affects 10–15% of your scalp hairs . Telogen hairs don't grow, but they don't fall out either. 2
This period lasts about 100 days for scalp hair and longer for brow, eyelash, arm, and leg hair. During this phase, the hair follicle rests and the club hair forms. It is now time to remove the hairs and reveal the root of the hairs. Every day, between 25 to 100 telogen hairs shed. 3
To preserve the hair's natural purpose, the epidermal cells lining the follicle channel continue to grow normally and may gather around the hair's base, temporarily anchoring it in place. 7
The follicle will re-grow, first weakening the shaft's anchor point. The hair base separates from the root and sheds. After two weeks, the new hair shaft will emerge from the telogen phase. The procedure causes shedding, or normal hair loss. 7
4. Exogen: Shedding Phase
Essentially, the exogen phase of hair growth is an extension of, or a portion of, the telogen stage of hair growth. Hair is shed from the scalp during the exogen phase, which is typically aided by washing and combing.6
During the exogen period, it is usual to lose between 50 and 100 hairs every day. As old hairs fall out of the follicles, new hairs start forming in the follicles during the exogen phase, which can last anywhere from 2 to 5 months.
On a daily basis, between 50 to 150 of your hairs may fall out. The rate of hair shedding is considered to be normal at this point.
What Happens when the Hair Growth Cycle is Disrupted? 4,9
You wouldn't lose all your hair at once if each hair follicle went through its growth cycle at its own pace. On a healthy head of hair, you shed between 80 and 100 hairs each day.
When your growth cycle is disturbed, hair loss, hair thinning, and issues with hair growth may develop. Metabolic abnormalities, disease, and poor diet can all lead to this.
Telogen effluvium, for example, may occur 12 weeks following a restrictive diet or a high fever (sudden diffuse hair fall). Hair fall increases three months after the anagen (growth) phase is cut short because numerous hairs enter the telogen (resting) phase at the same time.
The length of your hair may decrease if your hair development cycle is frequently disrupted (for example, if you don't eat a balanced diet). A lack of anagen phase time means that your hair can't grow at a length that's ideal for you.
We go through approximately 25 hair-growth cycles throughout our lifetime. It is crucial to remember that, despite the fact that hair goes through distinct phases, the overall quantity of hairs usually remains consistent throughout time. The ideal ratio between the Anagen and Telogen phases, known as the A/T ratio, is normally 80:20. However, this is rarely the case. This signifies that 80 percent of the hair is in the growing phase, while just 20 percent is in the falling phase.
How to maintain healthy hair growth?
Anagen phase hair growth is essential for optimal hair growth, hence hair should remain in this phase as long as feasible. A healthy diet and lifestyle, as well as employing products that care for your hair and preventing needless damage that may disturb your hair development cycle, can help you achieve this.5,6
The dermal papilla, which feeds the growing hair follicles, receives nutrients from the bloodstream, which then transports them via capillaries.
Your hair will not be affected by the events of today if you continue to lead a healthy lifestyle. Depending on when your hair is in its telogen phase, it may take many months before you start to notice a difference. Reduce stress, eat well, and avoid using heat styling products to protect your hair follicles from the harmful effects of heat.
References
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Hoover E, Alhajj M, Flores J. Physiology, Hair [Internet]. Ncbi.nlm.nih.gov. 2021 [cited 28 November 2021]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK499948/.
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Stages of Hair Growth Plus How to Maintain Hair Health in Every Stage [Internet]. Healthline. 2021 [cited 28 November 2021]. Available from: https://www.healthline.com/health/stages-of-hair-growth
- Help for Hair Loss [Internet]. WebMD. 2021 [cited 28 November 2021]. Available from: https://www.webmd.com/skin-problems-and-treatments/hair-loss/science-hair.
- Kingsley A. The Hair Growth Cycle: 4 Key Stages Explained | Philip Kingsley [Internet]. Philipkingsley.co.uk. 2021 [cited 28 November 2021]. Available from: https://www.philipkingsley.co.uk/hair-guide/hair-science/hair-growth-cycle.
- Ellis K. HAIR GROWTH CYCLE – THE 3 STAGES EXPLAINED [Internet]. 2021 [cited 28 November 2021]. Available from: https://www.growgorgeous.co.uk/blog/the-hair-lab/how-does-hair-grow/.
- Viviscal hair vitamin supplements for men and women [Internet]. Viviscal. 2021 [cited 28 November 2021]. Available from: https://www.viviscal.com/how-viviscal-works.
- Human hair growth - Wikipedia [Internet]. En.wikipedia.org. 2021 [cited 28 November 2021]. Available from: https://en.wikipedia.org/wiki/Human_hair_growth.
- Hair Anatomy: Overview, Microanatomy of Anagen Phase Hair, Microanatomy of Catagen Phase Hair [Internet]. Emedicine.medscape.com. 2021 [cited 28 November 2021]. Available from: https://emedicine.medscape.com/article/835470-overview#a2.
- The life cycle of your hair-Hair changes at different stages [Internet]. RichFeel ™. 2021 [cited 28 November 2021]. Available from: https://www.richfeel.com/hair-growth-cycle/.