Various research suggests that there is a seasonal rhythm to the hair cycle, with a peak of resting hair during the summer and greater shedding during the autumn. The causes of the phenomenon, however, are not yet fully known.
They call it “chestnut period”, and just as chestnuts and leaves fall, the hair also abandons our heads en masse. Is there anything to worry about? Absolutely not. This is a phenomenon totally physiologicalalthough there are few studies regarding the seasonality of hair loss. One of the most cited is the analysis of the University Hospital and the University of Zurich, which found a increase in resting hair around July. Since this phase lasts about 3 months, between September and October we find ourselves with a handful of hair in our hands. The reason, which is still unclear, could have to do with the shedding of animals: in autumn there would be greater hair loss to make room for new hair and protect the head from the winter cold. After all, winter is comingeven for our heads.
The three phases of the hair: anagen, catagen and telogen
Each hair follows a three-phase cycle: anagenthe growth phase, which can last years; catagena short phase of involution; And telogenthe rest phase. The latter lasts about 100 days, in which the mature hair remains anchored to the follicle but can be pushed away by the new growing hair (or by minimal mechanical stress) and fall out.
Knowing this cyclicality allows us, by counting how many hairs are in each phase at a given moment, to follow how the proportions change over the course of the year. In other words, to understand if, in addition to the rhythm of each individual hair, there is a seasonal one that unites them all. A question so widespread that it became the subject of an analysis of Google search data. In “Seasonal trends in hair loss: A big data analysis of Google search patterns and their association with seasonal factors“, the authors found that in the United States, between 2004 and 2020, research on hair loss becomes predominant in view of the autumn months.
There is a peak of resting hair in summer
The most extensive study on the topic, signed by Kunz, Seifert and Trüeb, comes from the University Hospital of Zurich. Between 2001 and 2007 researchers analyzed the trichogram (a technique in which the dermatologist tears with tweezers at least 50 hairs and count how many are in each phase) of women who complained of hair loss. After excluding women diagnosed with alopecia or taking medications that cause hair loss, the researchers analyzed the trichograms of well 823 women healthy.
From the data, the percentage of telogen hair peaked at Julywith a second, more modest peak in spring. Since the rest phase lasts approx 100 daysthe authors recall, the hair “stopped” in July would consequently fall out in autumn. The minimum amount of “resting” hair, however, arrives at the end of winter.
These results are actually nothing new, but they are still interesting due to the size of the sample. Of course, 823 women is still few, but it is still a fairly significant sample. The study, however, he didn’t follow the same people over the years. So, although it collected a considerable amount of data, it simply took a snapshot of what was happening over different months, on different women.
Various research confirms the annual hair cycle
The seasonal trend, however, is quite solid and also confirmed by smaller studies, such as that of Randall and Ebling, in 1991. Following 14 men for 18 months, the researchers saw that towards March, the number of growing follicles exceeded 90%, while in September the shedding increased.
With even more patience, Courtois and colleagues, in 1996, photographed the same portion of the scalp of 10 men over a period of up to 14 years. The result confirms the existence of an annual rhythm, with the peak of resting hair between the end of summer and the beginning of autumn, but more nuanced than Kunz’s study seems to indicate: in fact, the alternation of the seasons seems to only weigh about half on changes in the percentages of resting hair, while the other half would depend on other factors.
The causes and reasons for the autumn fall are not known
In animals it is common to change fur between winter and summer, we can also observe it at home with our cats, and this depends above all on the hours of light. According to many researchers, and as they also report Kunz, Courtois And Randall and Ebling even in human beings photoperiod and the shortening of the hours of light available, could regulate, through the hypothalamic-pituitary axis, the production of steroid and thyroid hormones, which in turn influence the follicle cycle.
Sometimes we read online that it is the stress we subject our hair to during the summer (heat, saltiness, tight hairstyles) that determines this greater fall in autumn. We can hypothesize, however, that since there is already more hair in the telogen phase and ready to fall out, adding stressful stimuli is not exactly a panacea. It’s a plausible explanation, for which, however, there are no concrete data.
What purpose would we need to molt? The Zurich group suggests that keeping our hair in winter protects us from the cold and postponing hair loss until the end of summer protects the scalp from the midday sun. But these are hypotheses yet to be verified.
