The summer before my junior year of college, I went to London with my sister. She’d planned a whole itinerary: Big Ben, high tea, the London Eye—and, for a touch of creepiness, the Jack the Ripper Tour. The tour, which traces the course of gruesome murders throughout the city, started off fine, if a little disturbing. But then at Mitre Square, while our guide excitedly detailed the unfortunate fate of Catherine Eddowe’s kidney (and uterus, and face), I lost vision. My temples throbbed with heat, cold sweat pearled on my limbs, and sounds became distorted echoes. When I came back to myself, I was slumped at the bottom of the pole I’d been leaning against, and the tour group—my sister included—was nowhere in sight.

I somehow caught up with the group and brushed off the episode as a fluke. That is, until we went to The Globe Theatre the next day to see Titus Andronicus, and I nearly lost consciousness when a character came onstage with her tongue cut off and “blood” pouring from her mouth. Afterward, adequate quantities of Indian food and Strongbow cider dulled the trauma of those moments, but that trip marked the beginning of years of similar episodes (including, last week, when I almost fainted on the subway while listening to a podcast host describe a toenail injury).

While the masses’ affinity for Game of Thrones suggests that people don’t generally faint at the sight of a lot of blood, it's common enough that there’s a name for it. Actually, there's a whole bunch of names: “vasovagal response,” “vagal response,” “neurally mediated syncope,” “neurocardiogenic syncope,” and “vasovagal syncope,” which is the term I’ve seen most often and the one I’ll use here. All of these names refer to signals being transmitted via the vagus nerve to the heart and blood vessels when a person freaks out (often at the sight of blood), resulting in lightheadedness or fainting.

“Vasovagal syncope can be initiated by a number of triggers—emotional stress, pain, fear,” says Rick Pescatore, director of clinical research at Crozer Keystone Health System in Pennsylvania. “In response to the trigger, the vagus nerve ‘fires,’ slowing the heart down and causing blood vessels to dilate.” This, in turn, means less blood makes it up to the brain, causing dizziness, vision and hearing loss, and fainting. Vasovagal syncope, then, is basically a misguided effort by the vagus nerve: In trying to make you chill out, it overshoots and instead makes you pass out.

What causes this misfire is less clear, though it has to do with something called “vagal tone,” or the functioning of the vagus nerve. “Problems occur when the balance between the parasympathetic and sympathetic systems gets disrupted,” says Jamal Benhamida, a New York-based pathologist, referring to the nervous system that spares the body from overusing energy and the one that controls a “fight or flight” stress reflex, respectively. "The vagus nerve becomes overactive, causing a rapid slowing of heart rate, which results in decreased blood getting pumped to the brain.” According to Pescatore, this may have been an evolutionary adaptation to staunch bleeding and decrease blood flow after an injury (“like turning down the spigot on a leaky hose,” as he describes it).

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“When you faint, you fall to the ground, which brings you back to consciousness,” says Jacob Lafo, a neuropsychology fellow at Brown University, describing a weird feature that our brain uses to protect you from staying unconscious too long. “It’s a protective mechanism that redistributes blood back up to the brain.”

Vasovagal is just one of many kinds of syncopes—turns out, people faint for a whole bunch of reasons. There’s carotid sinus syncope (fainting from sinus pressure), orthostatic syncope (fainting from dehydration), church syncope (fainting from standing up for too long), heat syncope (duh), and even micturition syncope (fainting during/after peeing). Still, vasovagal is by far the most common, making up about two-thirds of syncope presentations in the emergency department, Pescatore says. “However, its relationship to blood phobia is much rarer,” he says. “Less than 5 percent of people experience it.”

Beyond a genetic predisposition, a number of factors inform why, for instance, I get lightheaded during a gory episode of The Handmaid’s Tale while my husband doesn’t. For one, vasovagal syncope is more common in women, which likely relates to body size and heart rate, since women tend to have higher resting heart rates than men. Anecdotally, Lafo also observes that of the patients he's evaluated for vasovagal syncope, “many of the women were very petite, skinnier, had smaller builds. Very rarely would I be testing an overweight person.” This might help explains the fact that I, a 5’1” woman who’s 100 pounds soaking wet, can’t really hang when Offred rips part of her ear off, while my 6’7” husband continues to happily shove peanut butter M&Ms into his mouth, eyes glued to the screen.

Young people also tend to experience vasovagal syncope more frequently than their older counterparts, Benhamida tells me. “I’ve heard one theory that suggests that the nervous system is more responsive in youth, and it can respond rapidly—sometimes too rapidly,” he says.

So what’s a predisposed person in a trigger-filled world to do? Besides avoiding the triggers, stay hydrated and keep your electrolytes—which help regulate nerve and muscle function in the boy—up with salt or Gatorade. And if you start to feel vasovagal syncope coming on (and sitting isn’t an option) Lafo recommends engaging some muscles: “Interlock your hands and pull your arms in opposite directions, squeeze your biceps, contract your thighs. Any muscle contraction is good because it’s pumping blood back up toward your brain.” The worst thing you can do? Lean against something—like a pole—because this totally passive posture allows more blood to pool in your legs, setting you up to faint. Oops.

There’s another potential way to avoid vasovagal syncope: perceived control. In a 2015 study, participants were randomly divided into “perceived control” and “no perceived control” groups and then shown a video of mitral valve surgery, which is known to trigger vasovagal syncope. Those in the "perceived control" group were told they could decide when they wanted to take a break. The findings were pretty clear: Those who felt in control experienced significantly less vasovagal symptoms and anxiety. So if you’re watching a bloody movie with someone, maybe ask your companion if you can have permission to pause it if things get too intense—the initial research suggests that just knowing this is an option can make a big difference.

But all of these potentially mitigating factors are probably far less compelling than the real bottom line about vasovagal syncope. As Pescatore puts it, “while these episodes can be scary and disconcerting, they are almost categorically safe and after a short period of rest, the patient need not worry about anything sinister.”

Except, of course, Jack the Ripper. That dude was real.