Fall hair shedding, normal or not? Many people's hair grows and sheds on a yearly rhythm, and the most hair falls out in late summer and fall

The hair on your head doesn't grow all the time. Scientists split a hair's life into a growth phase and a resting phase, and hairs that go into rest fall out after a while. If there are fewer growing hairs in a given month, more hairs fall out.

The share of resting hairs changes with the seasons. Dermatologists looked at trichograms, a count of hairs by phase under a microscope, in 823 healthy women who had come in worried about shedding. The most resting hairs showed up in summer, a second, smaller peak came in spring, and the fewest were in late winter.

In the same 14 men from Sheffield, the share of growing hairs was highest in March, over 90%, and slowly dropped until September. The most hair fell out around August and September, right when the fewest hairs were growing. So after fall the rhythm turns back, and by spring the share of growing hairs is at its highest again.

Other researchers saw the same rhythm. In ten men followed for 8 to 14 years, the most resting hairs were in late summer and early fall. In 234 women who were worried about shedding, the number of hairs lost while washing went up in the fall, and the share of resting hairs started to rise a month earlier. In 41 people in China without hair loss, the share of resting hairs also rose around August and September, but less than in Europeans, and it stayed within the normal range. The studies of the ten men and of the people in China were done by employees of L'Oréal.

No one knows exactly why this happens. The authors of the study of ten men write that the rhythm is linked to climate and the number of hours of sunshine, but that's a link, not a proven cause. And in those of the ten men who had very few resting hairs, no rhythm was found at all. The author of one review on shedding suggests that strong summer sun is to blame and the shedding comes two or three months later, but that's only his idea.

Something similar shows up in internet searches. Scientists looked at Google searches, and according to the journal publisher's press release, people searched about hair loss more often in summer and fall. That tells us what people were interested in, not how much hair they lost, and the same press release says it's not known how seasonal shedding works.

To stop panicking over the hair in your brush, it helps to know what happens to a resting hair. DermNet, a reference site run by dermatologists in New Zealand, also writes that in a healthy person some hairs are growing while others are resting.

It also describes how a hair gets replaced. A new hair starts growing under the resting one and pushes it out. That's why DermNet counts up to 100 hairs a day in your brush, in the sink or on your pillow as normal. That's a description from a reference site, not a separate study.

The American Academy of Dermatology separates two things. Excessive shedding is when more hair than usual falls out, and hair loss is when something stops the hair from growing. A dermatologist can tell which one you have, and sometimes it's both at once.

Shedding can also have a delayed cause. The AAD writes that a few months after an illness, surgery or a hard time in your life, you may find a lot more hair in your brush and on your pillow. The NHS lists illness, stress, weight loss and low iron as causes of temporary shedding.

This kind of shedding usually passes. When the stress ends, the excessive shedding ends too, and according to the AAD, most people's hair gets back to its usual fullness in 6 to 9 months. But if the stress doesn't go away, the shedding can go on for a long time.

There's the most hair in the drain after you wash it, so it seems like washing is to blame. The American Academy of Dermatology writes that when people notice shedding, many change their part or hairstyle, buy products that promise to bring hair back and take hair growth supplements. Some women start washing their hair less often. According to the AAD, all this often helps only a little at best, and dermatologists tell their patients with shedding to use a mild shampoo.

Frequent washing has also been tested directly. In Xi'an, China, 1,500 healthy men and women without scalp conditions washed their hair the way they usually do for two weeks, and another 60 people took part in a five-week experiment. Overall, people were happy with their hair and scalp when they washed it 5 to 6 times a week. And the more often they washed it, the better, not worse, they rated their own shedding, and the difference was significant.

The authors write that worries about "overwashing" weren't confirmed. The study didn't count the hairs that fell out, though. It asked people how things felt. All costs were paid by Procter & Gamble, where two of the authors work.

A survey of 4,796 people from different clinics in Saudi Arabia came to the same conclusion. The more often people washed their hair, the better they rated the condition of their hair and scalp. But these are people's own answers, not an experiment.

So why is there so much hair on wash day? Think back to those 234 women. The middle of their range was 122 hairs in a single wash and 60 over a whole day, and in the fall even more hair came out while washing. DermNet names the sink as one of the usual places where hairs that fall out in the normal cycle end up.