Got a kid with a smartphone? New research shows it might just be cutting into sleep time.

It's a sign of the times that there are probably as many kids with smartphones in their bedrooms as there are kids with their own TVs. And while health experts have warned for years about the deleterious effects of screen time on a child's bedtime, they have a new foe in the smartphone, a gadget that not only has a screen, but also provides constant access to the outside world.

According to new research, children who slept in the same room as their tablets or smartphones got almost 21 fewer minutes of sleep than those without the gadgets. That's even more than the 18 minutes of sleep kids typically lose when they have TVs in their rooms.

The study tracked the behavior and sleep patterns of more than 2,000 elementary and middle school-aged-kids in two age groups, 9 and 12 years old. They found that those who slept in the same room with their gadgets reported fewer hours of sleep each night and also more feelings of drowsiness throughout the day.

"Studies have shown that traditional screens and screen time, like TV viewing, can interfere with sleep, but much less is known about the impacts of smartphones and other small screens," said lead author Jennifer Falbe of the School of Public Health at the University of California, Berkeley.

While parents used to worry about the glow of the screen from the TV or computer game keeping their kids awake, now they have to worry about the games, videos, websites, texts and Snapchats that kids feel compelled to access at all hours. And even if the sound is turned off, that vibrating hum of incoming calls, texts, and Instagram messages may keep kids awake.

According to the U.S. National Heart, Lung, and Blood Institute, elementary school-aged kids need at least 10 hours of sleep each day, while teenagers need between nine and 10. Yet many aren't getting nearly this amount, and the risks can't be overstated.

"The risks associated with shorter sleep duration and poorer sleep quality would include reduced academic performance, behavioral problems, possibly an increased risk for weight gain and possibly negative impacts on immunity," Falbe said.

The researchers also conceded that there could be other factors keeping these kids awake. For instance, there are generalities that could be made about the parents, lifestyle, or habits of kids with smartphones that might otherwise affect their sleep.

Still, this study joins a growing body of research suggesting that parents would be wise to limit their child's use of all screens — from TVs to computers to smartphones — when it's time to hit the hay.

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