Seasonal shedding: why more hair comes out in autumn

Every September, search traffic for hair loss climbs. Every September, a large number of people become convinced that something has gone wrong.

For most of them, nothing has. Human hair has a seasonal rhythm, it has been documented repeatedly, and autumn is when you find it in your drain.

What the research on seasonal hair shedding shows

Studies that follow healthy volunteers across a full year — counting hairs, or using trichograms to assess what proportion are in each phase of the growth cycle — have found a recurring pattern: the percentage of hairs in telogen, the resting phase, peaks around July and August.

Telogen lasts about three months. A hair that enters it in high summer is released in autumn. That delay is why the shedding appears in September, October and November, long after the trigger, and why it feels like it began for no reason.

The pattern shows up in more than one population and more than one study design. Some analyses also find a smaller secondary peak in spring. The effect is not enormous — it is a shift in proportions, not a moult — but on a head of 100,000 hairs, a few percentage points is a visible difference in a shower drain.

Why it happens is less settled than that it happens. The leading explanation involves photoperiod: day length is the most reliable seasonal signal an organism has, and hair cycling in mammals is strongly light-driven. Humans retain a vestige of that regulation. Others have suggested a protective function — more scalp coverage through the strongest sun months, shed when it is no longer needed.

Telling seasonal from serious

Seasonal shedding has a shape. Learning it saves a lot of anxiety.

It looks seasonal when:

• The increase is moderate — more in the brush and the drain, but the hair on your head does not visibly thin.
• It is diffuse, all over, with no patch or parting standing out.
• It arrives in autumn, and it happened around the same time last year.
• The hairs have the small white bulb of a completed cycle. The bulb check is the first thing to do.
• It settles within six to eight weeks.

It needs looking at when:

• The shedding is dramatic — handfuls, or a clearly visible reduction in density.
• It is patchy, or one specific area is thinning.
• It continues beyond two or three months.
• Your part has widened, or the hairs coming in are noticeably finer than they were.
• There are other symptoms: fatigue, weight change, cycle changes, cold intolerance, a sore or scaly scalp.

That last group belongs to a doctor. Iron deficiency and thyroid disorders are common, easily tested, and entirely invisible from the outside — and both cause exactly this kind of diffuse shedding.

The other three-month delay

Seasonal shedding is easy to confuse with telogen effluvium, which works on the same delay for a different reason: a stressor pushes a large group of follicles into telogen simultaneously, and three months later they are released together.

The distinction matters because one resolves on its own and the other has a cause worth finding. Practically: if the shedding is mild, in autumn, and matches last year, treat it as seasonal. If it is heavier and you can identify something that happened around three months ago — an illness, a birth, surgery, a crash diet, a bereavement — it is more likely effluvium, and the trigger is the thing to address.

What to do while it happens

Nothing dramatic, which is the hardest advice to follow.

Do not change your entire routine. The shedding started three months ago, before whatever you are about to blame. Changing five things now means you will never know what happened, and a new routine will get credit for a recovery that was always coming.

Do not brush harder to "get it over with". Hair already released will come out anyway; hair that is still attached will break.

Do count, if it helps you. Collect what you lose for a week and compare it with a week in a month's time. A trend is data; a bad shower is not.

Do photograph your part in the same light every couple of weeks. If the density is stable while the shedding feels dramatic, that is genuinely reassuring — and it is the reason to keep progress photos even when nothing seems to be happening.

Do get blood work if it lasts. Three months in with no resolution, ferritin and thyroid function are the standard first tests, and they are cheap.

The hardest part of seasonal shedding is that the reassurance and the resolution both arrive late. It is normal to be frightened by a shower drain. It is also, in October, usually just October.