*The following is excerpted from online articles posted on HealthDay.
Teens who are glued to their smartphones and other devices are unhappier than those who spend less time on digital media, new research finds.
The study can't prove cause-and-effect, so it's not clear if teens are made unhappy by spending a long time on their devices, or whether less happy teens are simply drawn to using them more.
But whatever the relationship, "the key to digital media use and happiness is limited use," believes study author Jean Twenge.
"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," said Twenge. She's professor of psychology at San Diego State University.
In the new research, Twenge's team surveyed more than a million 8th-, 10th- and 12th-graders across the United States. The study asked kids how much time they spent on their phones, tablets and computers; the amount of time they spent in face-to-face socializing; and their happiness levels.
On average, teens with higher levels of screen time were less happy than those who spent more time doing "non-screen" activities -- things such as sports, face-to-face time with others, and reading newspapers and magazines.
"Although this study can't show causation, several other [prior] studies have shown that more social media use leads to unhappiness, but unhappiness does not lead to more social media use," Twenge said in a university news release.
Just how much screen time is "healthy"? According to the research, the happiest teens in the study spent a little less than an hour a day scanning their smartphones, tablets or other devices. After that, levels of unhappiness tended to steadily increase with the amount of screen time.
The researchers also found that since the 1990s, increasing availability of screen devices was associated with an overall decline in U.S. teens' happiness.
Levels of life satisfaction, self-esteem and happiness among young people plummeted after 2012 -- the year that the percentage of Americans who owned a smartphone rose above 50 percent, Twenge noted.
The study was published in the journal Emotion.