Why does hair shedding increase in autumn?
― Hair that got "put on hold" in summer is falling out now
In early autumn, the hair in the bathtub drain or on your pillow can suddenly seem to multiply. This happens to many people every year, and it doesn't mean you're steadily losing hair. It's because each single hair is on its own separate clock for growing and shedding.
One September night, you wash your hair and lift the drain cover. Noticeably more hair than usual is tangled around it.
The next morning, a few more strands on the pillow. A whole clump on the hairbrush. It never bothered you in summer, and now you're worried.
But when you check the mirror, your hair doesn't look any thinner. So what's going on?
There are two main reasons
The hairs on your head don't all grow in sync. Each one cycles through a "growing period" and a "resting period" on its own. Only the hairs that have entered the resting period fall out, one after another.
Studies report that the proportion of hairs entering the resting period isn't constant across the year. It rises toward the end of summer, and those hairs fall out two to three months later — in other words, in autumn.
Put the two together, and you get a gap: what happened in summer becomes visible in autumn. Let's look at each step.
Hair doesn't keep growing forever
A single hair passes through three stages from birth to shedding. First comes a growing stage that lasts several years. Next comes a few weeks of transition. Finally, the hair stops growing and simply sits at the root — the "resting stage."
A hair in the resting stage no longer receives nourishment. It stays lodged in the scalp for several months. Then, as the next hair grows up from below, it gets pushed out and falls. By the time it falls, a new hair is already growing underneath it.
Look at the left side of Figure 1. It lays out the path a single hair follows, arranged in order from top to bottom. The growing stage is by far the longest; the resting stage is just a short stretch at the end. That's why, at any given moment, only a small fraction of all hairs are in the resting stage.
Decided in summer, seen in autumn
Multiple studies report that the share of hairs entering the resting phase shifts with the seasons. Surveys in the northern hemisphere have tended to find this share peaking toward the end of summer. The change in day length is thought to be involved, though no firm answer exists yet.
The key point is that nothing happens the moment a hair enters the resting phase. It's still there on your head. It falls only after passing through the resting phase — in other words, months later.
That's why, on the right of Figure 1, the upper solid line peaks in summer while the lower dashed line's peak sits further to the right. The shedding we notice in autumn is, in effect, a delayed notification of something that was already decided back in summer.
Dogs, cats, deer and other animals shed large amounts of fur all at once at the turn of a season. This is thought to happen because their bodies read the change in day length and swap winter coats for summer ones, or vice versa. Some researchers think human hair retains a faint echo of that same seasonal wave. But because human hair doesn't fall all at once, it never shows up as clearly as true moulting.
Even healthy people are said to shed 50 to 100 hairs a day. With around 100,000 hairs on the head, 100 is just one-thousandth of the total. Turning over one-thousandth a day barely changes how it looks. The raw count sounds alarming, but converted to a proportion, the picture changes.
Summary
Each hair grows, rests, and falls on its own separate clock. The share entering the resting phase rises at the end of summer, and those hairs actually fall months later. That's why shedding looks heavier in autumn. It's the regrowth queue getting a little more crowded than usual, nothing more.
Autumn shedding isn't the sound of your hair thinning out —
it's the sound of a queue that formed in summer, all coming due at once.
We've covered other stories about the body's rhythms syncing with the seasons and the clock elsewhere on the site. Take a look at "More articles" below.
- When you wash your hair, fit a fine-mesh net over the drain. Once you're done, spread the fallen hair out on a sheet of paper.
- Count it in bundles of ten and note the date. Every other day is fine too.
- Look at the roots of the fallen hairs in good light. If there's a small white bulb on the end, that hair completed its resting phase and fell naturally.
Keep it up for two to three months and your own record will show the autumn peak pass and the count settle back down. If the count doesn't fall, your scalp starts showing through, or hair falls out in patches, something other than the season may be behind it. Please consult a dermatologist.
Want to go deeper? ― Terms, formulas, and how this connects to textbooksWe mark clearly which level each part belongs to, from middle-school science through university specialist courses
- MSCovered in middle-school science
- HSCovered in high-school "Biology Basics / Biology"
- HS+High-school advanced content, or textbook sidebar material
- UnivNot covered in high school — university specialist content (dermatology, chronobiology)
- ResearchNot yet settled as textbook fact even at university — an active research question
MSTerms: this phenomenon has a name
- Hair cycle: the full loop a single hair goes through — growing, transitioning, resting, and falling. For scalp hair, this is said to take roughly two to six years.
- Anagen, catagen, telogen: the names of the hair cycle's three stages. They correspond to the "growing," "transition," and "resting" stages in the main text.
- Telogen effluvium: a state where the number of hairs entering the resting (telogen) phase temporarily rises, and they shed together a few months later. Besides seasonal change, this is also said to follow fevers or major physical stress.
- Moulting: many mammals swap their body fur at the turn of a season. The change in day length is thought to be the trigger.
MSHSCheck it with a formula: how many hairs a day counts as "normal"?
Given the total number of hairs, the share in the resting phase, and the length of the resting phase, you can get the number shed per day with nothing more than multiplication and division.
| Approximate number of hairs on the head | ~100,000 |
| Typical share in the resting phase | ~0.1 (10%) |
| Length of the resting phase | ~100 days |
| Hairs currently in the resting phase | 100,000 × 0.1 = 10,000 |
| Hairs shed per day | 10,000 ÷ 100 = 100 |
| Hairs shed per month | 100 × 30 = 3,000 |
Out comes the familiar figure of roughly 100 a day. Over a month that's 3,000 hairs — still just 3% of the total.
| When the resting-phase share rises to 15% | 100,000 × 0.15 = 15,000 |
| Daily count at that rate | 15,000 ÷ 100 = 150 |
| Difference from normal | 150 − 100 = 50 |
That's an increase of 50 hairs a day. At the drain, that shows up clearly as 1.5 times the usual amount — but on your head, it's only 0.05% of the total that has turned over. That's why the drain fills up while your appearance doesn't change.
| N | Total number of hairs on the head (unit: hairs) |
| P | Share of hairs in the resting phase (no unit, a number from 0 to 1) |
| T | Length of the resting phase (unit: days) |
| n | Hairs shed per day (unit: hairs per day) |
The relationship among these four can be stated as "multiply N by P, then divide by T, to get n." The unit works out to hairs divided by days — a per-day count.
HSHS+Why doesn't human hair shed all at once?
HSAt the root of each hair is a sac called the hair follicle. Cells dividing at its base push the hair upward. When division stops, the hair stops growing and enters the resting phase. Cell division and differentiation — concepts from Biology Basics — form the hair's entire life story.
HS+In the human scalp, each hair follicle runs on its own independent cycle. This is called an asynchronous pattern. In mice, by contrast, neighbouring follicles influence one another and their cycles progress together like a wave. It's this independence that keeps human hair from shedding all at once.
UnivHow does the body read day length?
Information about day length travels from the eyes to the brain's suprachiasmatic nucleus, which adjusts how long melatonin is released at night. In many mammals, this change in melatonin release is known to cue moulting and breeding seasons. Human hair follicles have also been reported to carry melatonin receptors, and researchers are investigating their link to seasonal waves. But because people live under indoor lighting, how much of the outdoor day-length signal actually reaches them varies a great deal.
ResearchWhat's still unclear
- The size and timing of the peak don't agree across studies Some studies report the resting-phase share peaking at the end of summer, while others find the peak's position shifting with latitude and sample size. Under which conditions the pattern reliably reproduces is not yet settled.
- Whether light or temperature is the trigger remains unresolved Day length, air temperature, and UV exposure all move together within the same season, which makes them hard to tease apart.
- How sex and age differences are handled Early studies were mostly done on men, and differences by sex or age group haven't been thoroughly examined.
So the content of this article, too, is "the best explanation we have for now." That shedding looks heavier in autumn is something many people experience, but exactly what causes it is still being debated.
How this connects to textbooks, by level
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Science: structure of the body | Where hair is made and where it falls out |
| HS | Biology Basics: cell division and differentiation | How hair is pushed up from the base of the follicle |
| HS+ | Biology: regulation of the internal environment | The asynchronous hair cycle vs. wave-synchronized patterns |
| Univ | Dermatology / chronobiology | Day length, melatonin, and reading the seasonal wave |
| Research | Hair follicle biology | Reproducibility of seasonal variation, and isolating the trigger |
| ― | Connection to daily life | Why a bigger drain count barely registers as a proportion |
- Randall VA, Ebling FJG, "Seasonal changes in human hair growth," British Journal of Dermatology, 1991 (reports seasonal variation in scalp hair growth and the resting-phase share)
- Kunz M, Seifert B, Trüeb RM, "Seasonality of hair shedding in healthy women complaining of hair loss," Dermatology, 2009 (examines seasonality in women reporting hair loss)
- Courtois M et al., "Periodicity in the growth and shedding of hair," British Journal of Dermatology, 1996 (long-term observation of the hair cycle)
- Japanese Dermatological Association, "Clinical Practice Guidelines for Male and Female Pattern Hair Loss" (男性型および女性型脱毛症診療ガイドライン, Japanese Dermatological Association) (covers the hair cycle and classification of hair loss)
- Paus R, Cotsarelis G, "The biology of hair follicles," New England Journal of Medicine, 1999 (review of follicle structure and the hair cycle)
※This article is a general-audience science explainer. The figures given are approximations meant to help illustrate the mechanism. If you're concerned about the amount of hair you're shedding or about your scalp becoming visible, please don't self-diagnose — consult a dermatologist or other medical professional.