In mid-March, schools across the country shut down. My seven-year-old son took it well; life went on. We spent some time early in the pandemic hiking, playing soccer, and even skiing a bit in Vermont, where we live.

As weeks of isolating ourselves turned into months, new behaviors emerged. First, it was sleep. By May, he wanted me or my wife to lie next to him to fall asleep. If he woke up in the middle of the night, he would come to get someone to sleep with him.

In June, activities that had brought him joy became battles to get started. Soccer? Basketball? Hiking? In Vermont, where the small population and ample outdoor space make social distancing easy, we were spending more and more time indoors. As July came, hissing and snarling took the place of words at the slightest disagreement.

And the kid who was first in line to hold scary reptiles at birthday parties is now afraid of bugs and runs away screaming at the sight of a caterpillar. It’s exhausting for him — and me.

I know I’m not alone in noticing these subtle (and sometimes not so subtle) changes in behavior and personality. Millions of other families whose kids are cut off from their schools, friends, and normal activities are experiencing similar shifts too. We say it’s an “emotionally hard time.” But that misses half of what emotions are. Kids’ bodies are being changed as they endure the constant stress and weight of the pandemic.

My company, Mightier, has a unique view of the change going on inside the bodies of so many children. The video game is an app-based, family program developed at Boston Children’s Hospital and Harvard Medical School that leverages biofeedback therapy to help children develop emotional strength: the ability to take on moments of challenge and frustration with grace. The games include a wearable monitor that measures the heart rates of players. That heart rate data is integrated into the games. As the player’s heart rate increases, the games get more challenging. Children are taught they need to take a breath, relax, and calm down before they continue, which builds emotional regulation skills and awareness.

This information has given us a unique view into the pandemic-driven changes going on inside the bodies of so many children and is helping us build a picture of how constant environmental stress and isolation can physically change kids. The remarkable thing we have seen during this pandemic is a rise in heart rate that looks sort of like a graph of the rise in Covid-19 cases over time.

The data aggregates the story of the pandemic among more than 17,000 children who use our video games for fun while building social, emotional, and coping skills. We looked at heart rate data collected from kids in the first week of playing a Mightier game before the game teaches them strategies to that help them manage frustration and therefore keep their heart rate low in gameplay., from July 2019 to July 2020.

The change from a historical average of between 90 and 90.5 beats per minute to more than 92 beats per minute is a significant and persistent change. Extrapolated over 24 hours, that represents an extra 2,000 heartbeats per child per day. On a purely physical level, kids’ hearts are working a lot harder than they were before the pandemic.

Kids often don’t have words for the changes they are feeling, but we can see them in their behaviors: poor sleep, less joy, and more undirected anger and frustration. Mental and physical health are often treated as two separate things, but there’s just one body, and the two feed each other.

As a parent during the pandemic, I find comfort in knowing that despite being isolated as we all practice social distancing, I am not alone in witnessing the variety of emotions and feelings my children are experiencing.  The pandemic has indeed stirred an intense, complex mental phenomenon that will likely grow in intensity over time. It’s a harsh reality knowing that ongoing outbreaks, lockdowns and school closures will likely continue to bring hidden changes to my son’s body (and mine). I recognize this as I struggle to martial yet another round of patience when another caterpillar dares cross our picnic table. My son’s body is being drained by the aftershocks of this pandemic.

I hope as we find increasing moments of normalcy, we put real effort into helping children overcome the stress, anxiety, and fear that come with the pandemic.

Jason Kahn is the cofounder and chief scientific officer of Mightier.