Adolescents using smartphones for more than four hours a day are at a higher risk of suffering from mental health issues, a South Korean study found Thursday.

The study by researchers at Hanyang University Medical Center analyzed the data of 54,809 teenagers across the country to find the correlation between daily smart phone usage time and adverse outcomes such as stress, depression, suicide, substance use and smartphone overdependence. They used the data from the state-conducted Korea Youth Risk Behavior Web-based Survey for 2017 and 2020.

It was found that the curvilinear relationship between smartphone usage time and adverse health outcomes were pronounced after four hours per day. While teens using smartphones between two and four hours a day showed signs of smartphone overdependence, they did not display increased adverse health outcomes compared to non-users.

"Using smartphones for less than four hours per day was significantly associated with stress perception, suicidal idealization and substance use after PSM (propensity score matching," the researchers wrote in the study. The research involved the propensity score matching to help account for other factors potentially linked to outcomes, gender, age and socioeconomic factors among others.

Compared to the four hours or under group, the four hours or above group logged higher propensity for stress perception by 16 percent, dissatisfaction with sleep by 17 percent, depressive symptoms by 22 percent, suicidal ideas by 22 percent, suicidal planning by 17 percent, suicidal attempt by 20 percent, alcohol problems by 66 percent and smoking by 90 percent. They were over twice as more likely to develop smartphone overdependence, and were 9 percent more likely to be subject to obesity.

But teens using smartphones between one to two hours a day were actually less likely to suffer from adverse health effects compared to those who do not use smartphones at all.

It was found that the general smartphone usage among teens increased in 2020 compared to 2017, as 85.7 percent of them in 2020 used the device for more than two hours a day compared to 64.3 percent in 2017. The percentage of those using smartphones for less than that was 35.7 percent in 2017, but plummeted to 14.4 percent in 2020.

"Our study demonstrated the curvilinear relationship between smartphone usage time and adverse health outcomes in adolescent. Our findings can help establish smartphone usage guidelines for adolescents," the researchers wrote.

The research was published in the open-access journal PLOS One.