Spending too much time using electronic devices during the day harms teens' sleep, a new study suggests.

The research included nearly 10,000 Norwegian teens, aged 16 to 19, who were asked how much screen time (computer, smartphone, tablet, video game console, television, MP3 player) they got during the day outside of school, and about the amount and quality of their sleep.

The use of any electronic device during the day and in the hour before bedtime was associated with an increased risk of taking longer than 60 minutes to fall asleep. In particular, the use of a computer, smartphone or MP3 player in the hour before bedtime was strongly linked with taking longer to fall asleep.

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It's important to note, however, that although the study tied use of these devices to sleep problems, it wasn't designed to prove whether or not the devices actually caused the sleep disruptions.

Dr. Mari Hysing, of the Regional Center for Child and Youth Mental Health and Child Welfare, Uni Research Health in Bergen, Norway, and colleagues published their findings online Feb. 2 in the journal BMJ Open.

The new study also found that daytime screen use of more than 4 hours was associated with a 49 percent higher risk of taking longer than 60 minutes to fall asleep. In addition, more than 2 hours of screen time after school was strongly linked with taking longer to fall asleep and shorter length of sleep.

On average, the teens said they needed 8 to 9 hours of sleep to feel rested. But the study found that teens who spent more than 2 hours emailing or chatting online had a more than a threefold risk of sleeping less than 5 hours. Teens who got more than 4 hours of any type of screen time were 3.5 times more likely to get less than 5 hours of sleep, according to the study.

When the researchers looked at individual devices, they found that computers had the strongest link with insufficient sleep, and were also the most widely used types of electronic devices.

Multitasking teens who used more than one device were more likely to take longer to get to sleep and to get less sleep than those who used only one device, the study authors noted in a journal news release. And teens who used four or more devices were 26 percent more likely to take an hour or more to fall asleep than those who used one device.

Compared to those who used one device, teens who used two to three devices were 50 percent more likely to sleep for less than 5 hours. Teens who used four or more devices were 75 percent more likely to sleep less than 5 hours a night, according to the study.

The researchers offered a number of explanations for their findings. The use of electronic devices may simply leave less time for sleep, the devices may interfere with sleep by stimulating the nervous system, or the light the devices emit may disrupt the body's internal clock.

The results show the need to update guidelines on teens' use of electronic devices, as well as provide guidance for parents on quantity and timing of electronic media use, the study authors said.

"The current recommendation is not to have a TV in the bedroom. It seems, however, that there may be other electronic devices exerting the same negative influence on sleep, such as PCs and mobile phones. The results confirm recommendations for restricting media use in general," the researchers concluded.