The times when I’ve pulled fistfuls of hair out of my head in the shower, my life wasn’t particularly stressful—or at least, it wasn’t until my hair started coming out. Then, of course, I assumed I was either balding or belatedly inheriting my family’s matrilineal thin hair. Faced with the stark reality of my hair-related vanity, I would try to comfort myself with the fact that I could at least wear fun wigs. You know, the natural cycle of things.

But the explanation for this actually lies in another cycle—the life cycle of a hair.

Basically, a hair grows, then stops growing, then falls out. (Revelatory, I know. But stay with me.) The growth phase, or anagen, of a human head hair can last two to six years, which is why our head hairs can get so long. Other hairs have much shorter growth phases—an eyelash’s, for example, is 30 days. After anagen, a hair goes into catagen, a very short phase lasting just a couple days, in which the follicle shrinks a little bit. That’s followed by telogen, when the hair pretty much just sits there, doing nothing. Then, at last, exogen, when it falls out.

This process is always happening, and it’s totally normal for a person to lose between 50 and 100 hairs a day this way. That’ll plug your drain up, but compared to the 90,000 to 150,000 total hairs on the average person’s head, it’s pennies.

Stress is thought to disrupt this process, prematurely kicking hairs out of the growth period. Rather than leaving anagen at their own pace, they all go through the resting phase at the same time and fall out together in bigger numbers—up to 10 times more than usual, according to the dermatologist Kurt Stenn, the author of the new book Hair: A Human History. This has been shown in mice, when the stress of being exposed to loud noises led their hairs to go into catagen prematurely.

“Stress does affect the human hair follicle as well, and we assume that what we see in the mouse is the same thing we see in the human—the same transition occurs,” Stenn says.

What exactly kicks the hairs into resting isn’t totally clear, but a review of the literature suggests neurotransmitters or hormones produced under stressful situations may be the cause. And a study done on rhesus macaque monkeys found that monkeys with more cortisol (a stress hormone) in their hair were more likely to have hair loss.

Because there’s a delay between when a hair stops growing and when it falls out, there’s likewise a delay between a stressful event (which can be physical, like surgery or trauma, or emotional, like a divorce or loss of a job) and when hair loss might occur.

In human head hair, this delay lasts three months, the combined length of the the catagen and telogen phases. It’s like clockwork. And indeed, those times when I was pulling my hair out in the shower, I was always able to trace it back three months to something atypically stressful, like a breakup or a death. (I asked Stenn if body hair falls out too. “That’s a wonderful question,” he said. “I don’t know that!” Even if it does, the resting period length is different for different kinds of hair.)

Mass shedding is also often seen after women give birth. (About three months after, most likely.) That’s thought to be hormone-related, Stenn writes—pregnant women have higher levels of hormones, which prevent shedding during pregnancy, and once they go away, so goes the hair. The American Academy of Dermatology ascribes it to “falling estrogen levels.” Giving birth is also, you know, a stressful event.

Post-pregnancy hair loss may be caused by a similar mechanism to other stress-induced hair loss, or it may not be. But “it’s a rule of thumb,” Stenn says, that if a patient comes into a dermatologist’s office worried about hair loss, the first thing they’ll ask is, “What happened three months before?”