What happens to your body when you watch a scary movie

When you watch a horror movie, you feel strange after, don't you? It's not like walking out of a comedy or drama when you've just seen a truly terrifying movie, one where you jumped a few times or let that awkward, nervous, I'm-not-scared-I-swear laughter slip out. (It's just a movie after all, right?) Scientists have found there are some very real reactions your body is having to what's playing out on the screen in front of you. Some people even seem to love horror movies for a weird biological reason…

You'll burn some calories

This bit of good news is for anyone who hates working out. According to a 2012 study from the University of Westminster (via The Telegraph), you can actually burn some calories while watching scary movies.

They measured volunteers' oxygen intake, carbon dioxide output, and heart rate to put together a picture of how much energy people were using to watch different horror movies. On average, they burned around 113 calories, which doesn't sound like much. It's not a ton, but it is the equivalent of what you'd burn during a 30-minute walk, which is better than nothing. You don't even have to put pants on.

Before you get too excited, there's a caveat. Movies had to be super-scary, and the best calorie-burners were the ones that made people jump. That's because the adrenaline rush is making your heart race, so a "meh" scary movie isn't going to cut it. In case you're curious, the study's top film was 1980's The Shining, which made viewers burn an average of 184 calories. Unsurprisingly, Jaws and The Exorcist rounded out the top three.

Your heart gets a literal workout

Your casual denial and nervous laughter might fool someone else into thinking you weren't really scared, but you can't fool your heart. That's been confirmed by a joint university study published in the medical journal Circulation, Arrhythmia and Electrophysiology (via University College London). When volunteers were shown scary film clips, researchers found there was a very definite change in blood pressure, heartbeat, and even in the cardiac muscles.

Shingo Murakami of Play.com (via Huffington Post UK) looked at the films 10,000 people ranked as the scariest, then hooked up audiences to heart-rate monitors and played the scariest ones. The top-ranked movies were The Shining, The Exorcist, and A Nightmare on Elm Street, and more fascinating than that is what those scary movies do to your heart. They found viewer's heart rates increased by an average of 25.3 percent, and that's the same escalation you see with a bit of light exercise. Sounds promising for getting a workout without getting off the couch, but UCL's Dr. Ben Hanson says it might be more complicated than that. It's such an increase it's entirely possible for someone with a weakened heart or other cardiac condition to have a seriously adverse reaction to a horror movie, and that might end your Netflix night with a real horror.

It's triggering your fight-or-flight response

At the same time you're yelling at the stupid heroine to run already, something is being triggered in your own brain: your fight-or-flight response. There's a reason you're so mad at the on-screen idiots who have clearly never seen a horror movie, and that's because your own basic, biological responses are putting you right into survival mode.

Psychologists from Concordia University looked at just what's going on in the brain while you're watching a horror movie, and the gorefest on the screen is setting off warning bells in some major parts of the brain, like the amygdala (which assesses threats and fears), the hippocampus (which provides context to stimuli), and the hypothalamus (which kick-starts your fight-or-flight response). We're not talking a tiny reaction, either; Popular Science says it's such a pronounced response that a San Diego neuromarketing firm called MindSign is starting to use the data from MRI scans to tell Hollywood how to build a better horror movie.

They're able to compare MRI activity to movie trailers, measuring such a primal response to on-screen imagery there's just no way for a person to lie about it. The amygdala in particular fires when there's a basic fear response, and they're hoping the data they collect will help create movies that will stick with you on a whole new level.

The fear is making you feel good

Fear and pleasure don't usually go together unless you're some kind of sociopath. We're not here to judge, but we will say Vanderbilt University professor Dr. David Zald has found that people who enjoy scary movies — and other fear-inducing things, like haunted houses — are wired just a bit differently.

Zald's research looked at how different brains received the pleasure chemical dopamine. People who like putting themselves in terrifying situations had fewer autoreceptors (which he describes as brakes) in their brains for dopamine, and Zald says this tolerance for fear likely played a role in our ancient survival. Being fearless made us better equipped to deal with all the dangers the world could throw at us, and you could argue it's not weird at all your movie tastes run along the lines of Revenge of the Surfboarding Killer Bikini Vampire Girls. You're just hard-wired for survival, that's all.

Sociologist Dr. Margee Kerr told The Atlantic there are a few other things going on here, too. Getting through a situation that scares your pants off gives you a major confidence boost at the end, and that feels good. You weirdo.

It's triggering a genetic reaction

Here's a nice bit of research you can cite the next time someone says there's something wrong with how much you like horror movies. According to a study from the University of Bonn (via The Telegraph), horror movies are actually acting on you and your body at a genetic level. The gene in play here is the COMT gene, which the U.S. National Library of Medicine says is responsible for helping dictate levels of certain transmitters in the brain and for directing how signals are transferred from one nerve to the next.

You can probably see where this is going. The university study found people who have two identical copies of the COMT gene were more susceptible to the actual, unadulterated fear a scary movie delivers. In other words, they're the ones screaming in the theater because their genetics are raising their anxiety levels and telling them they have every reason to be terrified. People who had two different versions of the gene were more likely to enjoy the movie and even laugh at some of the "scary" parts. You know those people. You might even be one. Maybe this is why you laugh at the goriest scenes. (Maybe.)

Your senses are heightened

Part of that fight-or-flight response involves triggering heightened senses. When researchers from the University of Wisconsin-Madison studied volunteers' reactions to horror movies, they found movie-watching subjects demonstrated increased eye movements along with an elevated heart rate and breathing. Georgia Tech researchers found volunteers who were watching suspenseful horror (specifically, Alfred Hitchcock's creeptastic films), tended to focus all their senses on the story during the most terrifying parts.

Writers and directors know this, too, and they exploit it. Charlie Lyne investigated how they capitalize on our biological reactions for his own film Fear Itself (via The Independent), and found horror movies are using plenty of tricks to get a very visceral response out of all your senses. Setting so many scenes during a dark and stormy night isn't just for atmosphere. The on-screen darkness actually makes your eyes move faster and amplify movement to make up for the color you can't see. When you do get color in horror movies, it's often red, for the simple reason it makes human brains uncomfortable. Sounds are the other big one, and Lyne says aside from the startling sounds (like horns and screechy music) you're aware of, some directors have started layering in low-frequency sounds just off our auditory range. You can't hear it, but you can feel it — and that's even worse.

You immune system goes into overdrive

First, a quick recap in case you slept through this day in science class. Leukocytes are white blood cells, and the U.S. National Library of Medicine says this group of cells are responsible for fighting disease and infection. There are lots of them, and they even govern our body's reaction to things like allergies. They're great, but watching scary movies puts them on red alert.

A study from Coventry University compared the leukocyte levels of a control group to a group that watched The Texas Chainsaw Massacre for the first time. They found that after watching the movie (the good version, from 1974), volunteers had more leukocytes circulating through their systems. They also had increased blood pressure and hemoglobin concentrations, suggesting that watching a truly terrifying movie can be so stressful your immune system is scrambling to fix it. There's no word on whether it'll help you fight that flu you feel coming on, but it couldn't hurt. And it has less sugar than orange juice.

Fear or arousal?

If you've ever gone to a scary movie on a date and then ended the night in the most successful way you can imagine, you might need to thank the movie rather than your charm. Sorry.

Inverse talked to Javid Sadr, a professor at the University of Lethbridge. He says the biological response your body has to a scary movie is pretty close to the same reaction you have when you're near someone you're attracted to. There's that increased heart rate, a release of adrenaline, and even dilated pupils. But the physical body doesn't actually connect emotions to a physical response, which means the fluttering heart you get from seeing a little girl projectile vomit a stream of pea soup into the face of a priest is almost the same fluttering heart you get when you see your true love across the room. Add in the fact that your body and brain aren't always on speaking terms, and the confusion between fear and arousal is so common there's a name for it: the misattribution of arousal. Of course, you could actually be attracted to the person sitting next to you … but you might want to think twice.

Kids react differently than you do

Dr. Margee Kerr is a sociologist and expert on fear. She's studied all kinds of fear from all kinds of sources, and told The Atlantic that she always warns parents to be vigilant about exposing their children to something scary. A child's malleable little mind can very easily latch onto something to create what's called a "flashbulb memory," and they can spend the rest of their lives traumatized by something that might even seem innocent to an adult. (She cites the case of Baby Albert, who was made to fear white rabbits. Hey, it was the 1920s.)

A study from the University of Michigan looked at the long-lasting impact horror movies can have on kids, and found 1 in 4 people surveyed still suffered from what they called residual anxiety. Childhood exposure to elements from horror movies — think the shark from Jaws — creates a life-long fear, and that can manifest itself in a range of ways from sweating, chills, and fever all the way to shaking, nausea, stomach pains, dizziness, difficulty breathing, and even a feeling of paralysis. It's worth remembering that even though you think letting Little Jenny stay up to watch Evil Dead is hilarious, she won't think so when she grows up afraid of trees.

Why do we enjoy all this?

Sociologist Margee Kerr also helps figure out how to scare people senseless in haunted houses, because some people just have all the luck. She's also looked at why we like being scared, and told The Hairpin it has something to do with the convergence of the biological reactions scary movies get, coupled with the fact we're feeling these frights in a safe environment where there's no real danger.

"Every night we turn on the news and hear all these depressing, scary things, and they just sort of sit there — there's no closure and little we can do about them," she said. "But when we go into a haunted house or watch a scary movie, we have that 'I did it' moment at the end, the big payoff. We feel like we faced something and beat it and came out on top."

And that's a hugely powerful feeling. (It's also pretty depressing, if you think about it.) She also said (via The Atlantic) that fear delivered via horror movie is a little different than real-life fears. The stimuli triggering your fight-or-flight response (and, in turn, small doses of dopamine) often happen at a lightning pace. That means our bodies react, but our brains process the fact there's no real danger just as fast. Turns out that fear is best when it's followed by nervous laughter and jokes about pants-wetting.