New U.S. research has added to the growing body of evidence that technology could be affecting our mental health, finding that teenagers who spend a large amount of time on their smartphone are more likely to be unhappy.

Carried out by researchers from San Diego State University and the University of Georgia, the study gathered data from surveys of 1.1 million 8th, 10th, and 12th graders across the U.S. who were asked about how much time they spent on their digital devices, their real-life social interactions with others, and their overall happiness.

On average, those who spent more time in front of screen devices, for example texting friends, playing computer games, or using social media, reported being less happy than those who spent more time partaking in non-screen activities such as sports, reading newspapers and magazines, and face-to-face social interaction.

Lead author Jean M. Twenge commented on the findings saying that an increase in screen time is the likely cause of this unhappiness, adding that, "Although this study can't show causation, several other studies have shown that more social media use leads to unhappiness, but unhappiness does not lead to more social media use."

The study also found that the drop in young people's life satisfaction, self-esteem and happiness which occurred after 2012 also coincided with the sudden increase in smartphone use and other screen devices, with 2012 the year that the percentage of Americans who owned a smartphone rose above 50 per cent, Twenge noted.

"By far the largest change in teens' lives between 2012 and 2016 was the increase in the amount of time they spent on digital media, and the subsequent decline in in-person social activities and sleep," she said, "The advent of the smartphone is the most plausible explanation for the sudden decrease in teens' psychological well-being."

However, quitting digital media altogether also doesn't seem to be the answer, with the team finding that those who spent a small amount of time in front of a screen -- a little less than an hour a day -- were actually the happiest. It was after this daily hour of screen time that unhappiness levels steadily rose as screen time also rose.

"The key to digital media use and happiness is limited use," Twenge said, "Aim to spend no more than two hours a day on digital media, and try to increase the amount of time you spend seeing friends face-to-face and exercising-two activities reliably linked to greater happiness."

The findings can be found published online in the journal Emotion.