Why do we lose so much hair?

Hair loss can be an upsetting experience. In a matter of weeks to months, a person can go from having a head full of luscious locks to a brush full of fallen strands.

A certain amount of hair shedding is normal; we lose an average of 50 to 100 strands of hair each day, according to the American Academy of Dermatology. But when is it considered abnormal? And what are some reasons for increased rates of hair loss?

Before diving into these questions, it's important to understand the hair cycle. Hair grows in four stages: anagen, when hair is actively growing; catagen, right before hair growth stops; telogen, when hair stops growing; and exogen, when hair sheds from the follicle. The anagen phase can last anywhere from two to eight years, catagen for a brief two weeks, and telogen for about three months before the strand sheds.

In an average person's lifetime, each hair follicle will undergo this cycle 10 to 30 times. As a person ages, the anagen or growth phase becomes shorter, and fewer hair follicles stay in the anagen phase.

Moreover, not all shedding is true hair loss. "A lot of times, hair loss is really hair breakage," Dr. Lynn McKinley-Grant, an associate professor of dermatology at Howard University College of Medicine, told Live Science. Breakage can happen from brushing hair too much or using hair styles that pull on the scalp, like weaves and tight braids. It can also happen from using harsh products or certain styling routines, McKinley-Grant said.

But shedding more hair than what grows back can be a sign of alopecia, the medical term for hair loss. There are many different types, ranging from hormone-induced hair loss to autoimmune disorders.

The list of potential causes is "enormous," Dr. Dan Baumgardt, an anatomist and primary care physician at the University of Bristol in the U.K. who advises patients on hair loss, told Live Science.

Related: Why don't overplucked eyebrows fully grow back?

Diet, extreme physical or emotional stress, hormonal changes, autoimmune disorders, certain medications and scalp health can all affect the amount and type of hair loss. Often, more than one of these factors are at play. In a type of alopecia called telogen effluvium, some hair very suddenly sheds across the entire scalp. Major surgeries, bad cases of the flu or COVID-19, chemotherapy, diabetes, anemia, thyroid disorders, hormone fluctuations, and a slew of other health conditions can trigger this type of hair loss. Hormonal changes during pregnancy can pause the shedding phase but then trigger telogen effluvium during the postpartum period.

With a limited amount of available energy at any given time, the body may stop hair from growing so it can spend energy elsewhere when we are sick or otherwise stressed, McKinley-Grant said. It's usually not until three to six months later, after more hair from the resting telogen phase has fallen out, that people tend to notice it. Most of the time, this hair will grow back, she said.

Other common forms of alopecia are male- and female-pattern hair loss. The former is characterized by a receding hairline, and the latter by overall thinning and a widening hair part. Both men and women can have either of these hair-loss patterns, depending on the levels of certain hormones.

Scalp health can also affect how much hair we shed. Scalp inflammation, such as that caused by psoriasis, can stop hair from growing, but it can be treated, McKinley-Grant said. Lesser known conditions like seborrheic dermatitis, a type of fungal disease, can cause inflammation, too. Because this makes the scalp itchy, a person's scratching can damage hair follicles.

People with longer hair might notice more hair shedding than people with shorter hair — for example, on hairbrushes — simply because it's easier to see. But noticing a thinner ponytail, round bald patches, or a receding hairline can be reasons to see a doctor, McKinley-Grant added.

"Skin, hair and nails really reflect your overall health," McKinley-Grant said, making dermatologists well positioned to help identify underlying health conditions such as heart and liver problems, anemia and joint inflammation. "Skin is the largest organ of the body, so the blood goes through all the organs and stuff lands up in the skin." In this sense, hair health can be a helpful indicator of overall health for practitioners.

There are plenty of products marketed on social media for hair growth, but it's important to talk to a doctor first. Oftentimes, those brands tout results based on faulty methods, Baumgardt said, such as using patient surveys rather than actual science. "I wouldn't do any of them until you talk to a dermatologist," McKinley-Grant added.

This article is for informational purposes only and is not meant to offer medical advice.