ScienceDaily summarizes the collaborative research of Stockholm University, Uppsala University and the Karolinska Institute thusly:

In the study boys (12-15 [years old]) were asked to play two different video games at home in the evening. The boys' heart rate was registered, among other parameters. It turned out that the heart rate variability was affected to a higher degree when the boys were playing games focusing on violence compared with games without violent features. Differences in heart rate variability were registered both while the boys were playing the games and when they were sleeping that night.

Heart rate variability is a medical term for the regularity of a heartbeat, meaning that a low variability signals the heart is working smoothly and things are generally well. Meanwhile, a high variability is a precursor to irregular heartbeats, which are symptomatic of certain medical problems, high levels of stress, or strongly negative emotions, according to the research firm Institute of HeartMath.

In recent research, heart rate variability has been linked to the nervous system, which is why mental activities like stress or violence can contribute to increasing that variability.

The researchers aim to continue studying the effects of video games on the body and hope that their findings will contribute to determining whether video game violence causes real-life violence.