A woman caught her boyfriend cheating when his Fitbit activity spiked at 4 a.m.

• Last week, a woman tweeted that she once caught a boyfriend cheating when his Fitbit recorded a 4 a.m. spike in physical activity. The tweet went viral, prompting others to share similar stories.
• Sex can indeed raise your heart rate, burn calories, and show up on fitness tracking devicees, according to an exercise science expert. 
• However, there are less scandalous and even potentially dangerous reasons for middle-of-the-night heart rate spikes, like nightmares and heart attacks.
• Visit Insider's homepage for more. 

NFL Reporter Jane Slater went viral last week after sharing her story of fitness gift-giving gone wrong.

In response to a viral Peloton ad, widely criticized for its tone-deaf portrayal of what some interpret as a husband urging his thin wife to work out more, Slater tweeted about a now-ex-boyfriend who got her a Fitbit fitness tracker for Christmas.

The couple synced their devices to motivate each other to get in shape, she said, but a 4 a.m. spike in his physical activity alerted Slater that he was up to more than just a late-night, or early morning, jog. 

"Spoiler alert: he was not enrolled in an OrangeTheory class at 4am," she tweeted about discovering his infidelity. 

Slater's story inspired a few others to share similar accounts of fitness apps tracking illicit extracurriculars. 

It turns out, sex does show up on a fitness tracker, according to Alex Koch, professor of exercise science at Lenoir-Rhyne University in North Carolina.

He told Insider that Fitbits and similar devices have a very accurate heart rate monitor that would capture a spike due to vigorous activity. It also has a accelerometer, a motion-detecting device that picks up vibrations from movement, which would include the horizontal tango.

Sex raises your heart rate and burns calories

Like any physical activity, sex engages your muscles, including your heart, and would show up on a fitness monitor as a spike in heart rate. "There's definitely going to be a substantial rise in heart rate from resting, depending on how energetic you are," he said.

He would know. Koch said he once accidentally left his Apple Watch on during sex with his wife, who gave him permission to tell the story to Insider. The resulting activity was interpreted as climbing several flights of stairs. 

Sex also burns calories — exactly how many depends on many factors, including age, weight, gender, the intensity of the activity, and the position you're having sex in, according to Koch.  

For the average person, though, sex burns between 4 to 7 calories a minute, according to the most recent data on physical activities from Arizona State University. 

While this is a great perk of love-making, it doesn't mean sex wins out over more traditional workouts, Koch said. 

"More sex is better for a lot of reasons, including for emotional well-being and as a core workout, but probably isn't the best way to burn a lot of calories," he said. 

But mysterious Fitbit activity doesn't necessarily mean your partner is cheating

Although a Fitbit could pick up sexual activity, it wouldn't be able to distinguish it from other types of physical exercise, Koch said. Even a 4 a.m. spike in heart rate, while suspicious, isn't always proof your partner is cheating. 

"It [detects] any kind of movement, so there could be some innocent explanations," Koch said.

For instance, a strenuous round of masturbation could cause the same kind of jump in activity as partnered sex, he explained.

So could having a nightmare or sleepwalking. Even less pleasant events, like vomiting or other gastrointestinal distress, could raise your heart rate enough to look unusual on a tracking device, as could a serious medical event like a heart attack.

That's why context clues make all the difference in deciding how suspicious the activity really is, Koch said. Regular late-night activity would be much harder to explain than a single occasion. After all, he said, people don't have heart attacks "every single night," 

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