More than two hours of screen time after school was strongly linked to both delayed and shorter sleep.Almost all the teens from Norway said they used the devices shortly before going to bed.Many said they often got less than five hours sleep a night.

　　The teens were asked questions about their sleep routine on weekdays and at weekends，as well as how much screen time they clocked up outside school hours.On average，girls said they spent around five and a half hours a day watching TV or using computers，smartphones or other electronic devices.And boys spent slightly more time in front of a screen—around six and a half hours a day，on average.Playing computer games was more popular among the boys，whereas girls were more likely to spend their time chatting online!

　　Any type of screen use during the day and in the hour before bedtime appeared to disturb sleep—making it more difficult for teenagers to nod_off.And the more hours they spent on these devices，the more disturbed their sleep became.When daytime screen use totalled four or more hours，teens had a 49% greater risk of taking longer than an hour to fall asleep.These teens also tended to get less than five hours of sleep per night.

　　Dr.Hysing said her findings had suggestions for the wider population as so many people use these devices.“We know that sufficient sleep is essential for good physical and mental health.Logging off may be one important step toward securing a good night’s sleep.”

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