A kid reading(Getty Images)

If you're like many mothers of tweens or teens, you probably often find yourself gazing at the faces of your beloved children — as those faces are bathed in the flickering light of the television or the cool glow of the computer. Observing their glassy-eyed stares, it's hard not to wonder (and worry), What is this screen time doing to their development?

We've all heard the recommendation of the American Academy of Pediatrics: just one to two hours a day of "quality" electronic entertainment for children over 2. Yeah, right. In 2010, a study in the American Journal of Epidemiology reported that most adolescents spend an average of 25 to 30 hours per week watching TV and using computers. And while half of parents surveyed in a 2010 study said they always or often set limits on screen time, 18 percent of their kids really disagreed.

"It's getting more complicated to measure how much screen time kids are getting," notes Lisa Guernsey, author of Into the Minds of Babes, a book about children's use of electronic media. "We're no longer talking about the TV in the den that parents can turn off. These days, many teens and tweens have smartphones, laptops, tablets, and iPods that they carry with them." When you add up the total time kids spend on their electronic devices, you arrive at a truly staggering number: The average American between the ages of 8 and 18 spends more than seven hours a day looking at a screen of some kind, reports a Kaiser Family Foundation study. "When we conducted a similar survey five years before, we thought children's screen time couldn't rise any higher," says Donald F. Roberts, Ph.D., a Stanford University communications professor who coauthored the study. "But it just keeps going up and up."

Scientists are now beginning to tease out the effects of all this electronic engagement. Too much screen time may be linked to an increased incidence of risky behaviors, and more social network activity seems to correspond to mood problems among teens. But there's good news, too. Moderate computer use may be associated with the development of some cognitive and social skills. Here, a closer look at the cons and then the pros of screen time:

WHEN TO WORRY
The more hours teenagers spend using a computer or watching TV, the weaker their emotional bonds with their parents, reports a study of more than 3,000 adolescents published in the Archives of Pediatrics & Adolescent Medicine. On the other hand, teens who spent more time reading and doing homework reported feeling closer to their moms and dads. "Strong attachment to parents" — a bond of understanding, trust, and affection — "is protective against poor psychological health and participation in risky health behaviors," the study's authors note, so "concern about high levels of screen time is warranted."

• No surprise here: Screen time can make a kid fat. Kelly Laurson, Ph.D., a professor of kinesiology and recreation at Illinois State University, asked more than 700 children to wear pedometers and report how much time they spent watching TV and playing video games. He found that a lack of exercise and a surfeit of screen time each contributed to kids' growing girth. "Kids are more likely to eat when in front of the TV, and TV shows lots of ads for unhealthy foods," says Laurson. "Too much screen use also interferes with sleep. These influences can make kids fatter and less fit, even if they are physically active."

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• Researchers at Queen's University in Canada found that youths with the highest level of computer use (more than three to four hours a day) were 50% more likely to engage in risky behaviors such as drinking, smoking, drug use, and unprotected sex than kids with minimal amounts. "More and more advertising has moved to the Web, and these ads are far less regulated than those on TV," says researcher Valerie Carson. "Kids who use computers can be exposed to many examples of dangerous behaviors, which they may then emulate."

If you feel tempted right about now to declare a complete ban on screen time, consider this: Research also suggests that playing video games or visiting social-networking sites like Facebook may produce improvements in certain skills.