Our hair is always growing and falling and growing and falling. We shed close to 100 strands of hair per day, and yet our hair continues to lengthen. But how does it work? We turned to board-certified dermatologist Dr. Sonia Batra, MSc, MPH, MD—who is also the Medical Advisor for hair care brand Keranique—for more information. First things first: “Hair grows in a cycle,” says Dr. Batra, “but each follicle functions as an independent unit so that hair is not shed all at once.” Below, she breaks down each phase of the hair cycle and what could affect hair growth.
At any given time, each strand of hair may be in one of the four phases of growth. Dr. Batra explains the differences:
Anagen: The growth phase, in which the hair cells actively divide. On the scalp, the average period is 3–4 years.
Catagen: This is a transitional phase, in which the cells are no longer dividing and hair regresses to form a club hair that moves closer towards the surface of the scalp. This usually lasts approximately 2 weeks.
Telogen: A resting phase during which the regressed or club hair remains in the follicle. This usually lasts 1–4 months.
Exogen: Exogen used to be considered a part of telogen, but more recently has been recognized as a separate phase in which a new anagen hair forms in the follicle and dislodges the prior resting hair, such that the old hair is shed and a new follicle starts to grow in its place. The average person loses 50–100 hairs per day.
“As we age, the percentage of hair in the growth cycle decreases,” says Dr. Batra. She notes that “it decreases even faster in people with hereditary baldness (androgenetic alopecia or female pattern hair loss).” Additionally, your hair cycle might be affected by environmental stressors. Dr. Batra explains: “Environmental stressors such as medications or chemotherapy can interfere with the cell division that allow growth of the hair. This leads to a thin shaft which breaks within the follicle.”
Related Reading: Who is most at risk for thinning hair?
“Usually 85–90 percent of all scalp hairs are in anagen phase, 1 percent are regressing and the remaining percent are resting or being shed,” says Dr. Batra. This is why you’ll typically see an average of 50–100 hairs falling out per day.
Related Reading: How much hair fall is normal?
“On the scalp, a half inch per month (approximately 1cm) is the average rate of growth,” says Dr. Batra. However, this doesn’t mean that your hair can’t grow faster or slower than that. She notes that anywhere from 0.6cm to 3.36cm is considered normal. “This rate can vary depending on age, hormones, genetics and illness,” Dr. Batra explains. And to support a healthy follicle and normal hair growth, she recommends “a diet with adequate iron, protein and vitamin B.”
Related Reading: How to keep your hair healthy and strong
If your hair is suddenly shedding more than normal or you’re starting to see patches of total hair loss, you may want to talk to your doctor about potential causes—as the treatment options may differ. For example, as Dr. Batra mentions, certain medications, chemotherapy and even emotional distress can lead to sudden thinning. On the other hand, female pattern baldness is a genetic cause, resulting in a more gradual thinning.
The treatment for each of these differs—ranging from over-the-counter shampoos and topical treatments to changing your environment. Some of the most common ways to stimulate hair growth include using vitamin-based shampoos, applying treatments containing minoxidil or aminexil or adjusting your diet to promote healthy hair.
Related Reading: 4 ways to treat hair loss
Additional References, From Dr. Batra: