Why Your Hair Sheds More in Fall: The Seasonal Variation Explained

More hair in the shower in September and October. Less visible change in spring. This isn't imagination — there's solid data on seasonal hair cycling, and understanding it is what separates "my treatment stopped working" from "this is a completely normal annual pattern."

01The data on seasonal variation

A 2009 study analyzing hair loss patterns across multiple populations found a statistically significant peak in the proportion of telogen-phase hairs in late summer (August), with a resulting shed peaking in fall (September–October). The proposed mechanism involves photoperiod — changing day length affects melatonin and prolactin, which modulate the hair cycle. Unlike seasonal coat cycles in animals, human seasonal variation is subtle but measurable.

02Why it feels worse in AGA patients

People with AGA have a higher baseline proportion of follicles in telogen and transitional phases already. When seasonal variation adds another 5–10% telogen shift on top of that baseline, the visible shedding is disproportionately alarming — more hair, fewer hairs to lose, the math gets dramatic quickly. The seasonal component itself is the same size; the starting position makes it more visible.

03The vitamin D connection

Vitamin D levels drop in fall and winter in northern latitudes — the same period as the shed peak. Vitamin D has documented receptors in hair follicles and deficiency has its own hair cycle effects. A vitamin D supplement through fall and winter is a cheap, low-risk intervention that addresses this seasonal variable regardless of whether it's the primary driver.