Here's What Goes On Inside Your Body When You Watch A Scary Movie

Horror movies can do a number on your nervous system, but are they actually so spooky that they can affect your health?

We've all experienced those jumpy, heart-racing moments while watching a scary movie — and the scarier it is, the more our body reacts.

Most of us can probably remember that one horror movie or scene that made us jump out of our seats and haunted us for the rest of our childhood. Maybe it was when the little girl crawls out of the TV in The Ring or when the alien runs through the corn maze in Signs. Maybe — like for me — it was the shark attack scene in Jaws.

Scary movies have a way of making us scream, panic, and sweat even though we know we aren't in real danger. Some people love the rush and others avoid it at all costs.

The makers of new film Hereditary recently tried to prove this theory by asking viewers to monitor their heart rate with an Apple Watch during the film.

please look at my heart rate at the beginning of hereditary vs. my heart rate during that scene in hereditary

Hereditary, a dark, twisted story of family tragedy, is already being hailed as one of the most disturbing horror movies in recent times. (Check out our review, "Can A Movie Be Too Scary? "Hereditary" Might Come Close.") To put it simply, the movie will mess you up. Watching it has become somewhat of a challenge — more specifically, a "heart rate challenge."

A24, the company behind the film, had select screenings around the country where they asked audience members to record their heart rates during the movie using the health-tracking app on an Apple Watch. The brilliant promotional strategy showed that viewers had spikes in their heart rate as high as 140, 157, and 164 beats per minute (BPM) — which is high for an adult (we'll put these numbers in context in a little bit). Adrenaline junkies, rejoice.

Monitoring devices like the Apple Watch or Fitbit aren't always 100% accurate, so the science isn't perfect. But A24's "heart rate challenge" is certainly a reminder that terrifying movies can cause real physical changes in our body.

But how exactly do scary movies affect the heart and body, and are there any health risks?

We spoke to an expert, Dr. Regis Fernandes, a cardiologist at the Mayo Clinic in Phoenix, Arizona, to find out what actually goes on inside the body when you watch a scary movie.

Scary movies can trigger the body's "fight or flight" response, which increases your heart rate and blood pressure.

The "fight or flight" is a physiological response that both humans and animals have evolved over time to help us stay alive. When we are exposed to danger or perceive a threat, this response is activated so that we have the energy to move away or fight back — fast. "The response can also be activated with anxiety — so the danger might not exist but your mind creates the same feeling," Fernandes told BuzzFeed News.

When we are in fear of danger, the body alerts the amygdala, a part of the brain that deals with emotional response. The amygdala then sends a message to the hypothalamus, which tells the body to release adrenaline — a hormone that helps modulate some of this fight or flight response.

All of this causes your sympathetic nervous system to ramp up and increase your heart rate, blood pressure, and blood flow to muscles. "Essentially, it's preparing your body for exercise," Fernandes said. You may even start to sweat a little and breathe faster, too.

"It's a very basic response, meaning we can't really control it because it's an instinct," Fernandes said. Scary movies can trick our minds into believing the dangerous situation we are watching unfold onscreen poses a threat in real life. So even if you know you are safe on your couch or in a movie theater seat, your body still reacts as if Freddy Krueger is coming at you with the knife gloves.

Generally, the increase in heart rate you get from a scary movie is similar to the increase you'd experience during mild to moderate exercise.

In healthy adults, the resting heart rate is normally between 60 and 100 BPM. During the Hereditary screenings, spikes were between 140–160 BPM, which are big leaps but also pretty much expected for most adults. Here's why.

"I would say movies would not cause the heart to beat faster than mild- to moderate-intensity exercise, or about 70–85% of your maximal heart rate," said Fernandes. Your maximal heart rate is calculated by subtracting your age from 220 — and this number is the upper limit of intensity your heart should reach during exercise. So for a 30-year-old, that would mean 190 BPM is the maximal heart rate and a scary movie might be expected to send your heart rate to spike around 130–160 BPM.

Of course, some people will be less scared than others and their heart rate won't have a huge increase. It also depends on your age, resting heart rate, and your fitness level. "When we talk about heart rate, it's very individual — 150 BPM for one person can mean something very different for another," Fernandes said. So whether a 164 BPM spike is "too high" depends on the person.

And no, this doesn't mean watching a scary movie is equivalent to exercising. We know you were secretly wondering.

Your muscles might also tense up and contract.

Have you ever noticed that during a scary movie, you might clench your limbs against your body, curl into a ball, and grip the armrests or blanket in front of your face so hard your knuckles turn white? Muscle tension is another response to perceived threat. "Fear can cause your muscles to contract and tense up because this part of your body is preparing for a reaction," Fernandes said.

When you know a scary scene is approaching and you feel that sense of dread and looming threat, your muscles will tighten up until the action is over. You might even hold your breath. "Once the scary scene is over, your muscles will relax again," Fernandes said. All of the contracting and relaxing might make you feel a little worn out by the time the movie is over.

So for the overwhelming majority of people, the changes that occur in the body from watching a scary movie are not bad for your health.

You may have heard anecdotes about people having heart attacks or collapsing in theaters from watching horror movies like The Exorcist. While these events are theoretically possible if the person has an underlying heart condition, they are extremely unlikely, Fernandes said — and there's not really any scientific evidence that a movie can "scare you to death."

People with preexisting conditions or heart problems should avoid increasing their heart rate above a certain level from any activity — whether that's from a scary movie, exercise, or sex (yes, sex), it doesn't matter. "If people have a tendency to not tolerate a high heart rate or blood pressure, then yes, this fight or flight response may not be healthy for them," Fernandes said.

That being said, most of these people can usually still tolerate exercise at a mild or moderate level — which, as we mentioned, increases your heart rate to the same level as a scary movie, said Fernandes. If you are really worried about your heart and scary movies, talk to your cardiologist or primary care provider.

So for most people, the spikes in heart rate and blood pressure from watching a scary movie are not a problem. Your body will return to normal, unharmed, after the movie ends.

As for the psychological harm from horror films... Well, that's a whole different story.

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