Teenagers who are constantly glued to their smartphones or tablets are more unhappy than those who go and do something else, a new study found.
Those aged 13 to 18 who spend the most time whiling away their time playing computer games, on social media, texting or video chatting were less happy than those who go and play sports, reading newspapers and magazines, or meet friends.
But kids who shunned using the gadgets are not happy either.
In fact the happiest teens were those who limited their daily digital media time to a little less than an hour a day.
Yet after this daily hour of screen time, unhappiness rises steadily along with increasing screen time.
And the rise in the gadgets since the Millennium has coincided with American teens feeling less satisfied with life and having lower self-esteem and happiness than those who grew up in the 1990s before the gadgets became ubiquitous.
Professor of psychology Jean Twenge at San Diego State University said: "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 key to digital media use and happiness is limited use.
"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."
Together with colleagues Gabrielle Martin at SDSU and Keith Campbell at the University of Georgia they looked at data from the Monitoring the Future (MtF) longitudinal study.
This was a nationally representative survey of more than a million school children aged 13 to 14, 15 to 16 and 17 to 18.
The pupils were asked how often they spent time on their phones, tablets and computers, as well as questions about their in-the-flesh social interactions and their overall happiness.
Professor Twenge added screen time was driving unhappiness rather than the other way around.
Looking at historical trends from the same age groups since the 1990s, the researchers found the proliferation of screen devices over time coincided with a general drop-off in reported happiness in American teens.
Specifically since 2012 their life satisfaction, self-esteem and happiness plummeted and that year marked the point when the percentage of Americans who owned a smartphone rose above 50 per cent.
Prof Twenge said: "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.
"The advent of the smartphone is the most plausible explanation for the sudden decrease in teens' psychological well-being."
The study was published in the journal Emotion.