Nearly 6,600 12 to 15-year-old Americans self-reported how much time they spent per day on social media, as well as whether they had any mental health problems. The researchers found that three hours of social media correlated with higher rates of mental health issues, even after adjusting for a history of such problems.

The effects of social media consumption on teens manifest in two main ways, according to the study’s authors: internally and externally. The latter was essentially nonexistent among teens who reported that they didn’t use social media.

Researchers have long struggled with understanding how social media, screen time, and other forms of personal technology affect child and adolescent brain development. Much of that is because technology develops faster than it can be studied. It also doesn’t help that researchers have come to conflicting conclusions. 

For example, this study from last month at the University of California, Irvine, suggested that there was no link between tech time and mental health. Social media, however, might be different: this study published earlier this year found a worrying link between social-media use and social-media addiction.

Kira Riehm, a Ph. D. student at Johns Hopkins and the lead author of the study, says the three-hour cutoff shouldn’t be taken as a concrete rule. 

Future studies could track, in real-time, the amount of time spent on social media use, which would provide more precision in estimating some sort of cutoff. It’s the study’s broader point that Riehm says is worth remembering: excessive time on social media doesn’t help people’s mental state.