Parents have long suspected it, but now doctors have proof: the more time teenagers spend on computers or mobile phones, the less they sleep, especially if the gadget is used just before bedtime.
The evidence is so strong that health watchdogs should overhaul guidelines for electronic device use by youngsters, the experts said.
The team surveyed nearly 10,000 people aged 16 to 19 in Norway in 2012. Teens were questioned about their sleeping patterns, how long they looked at a screen outside of school hours and the type of gadget they used.
The respondents said they needed between eight and nine hours' sleep on average to feel rested.
Those with screen time of more than four hours a day were three-and-a-half times likelier to sleep fewer than five hours at night.
The study also confirmed what many parents of a sleepy teen have experienced already – using an electronic device in the hour before bedtime badly affects both onset of sleep and its duration.
Teens who used a computer or mobile phone in the last hour before bed were about one-and-a-half times more likely to take more than 60 minutes to fall asleep. Adults normally nod off in under 30 minutes.
Somewhat smaller risks of delayed or shortened sleep were observed amongst youngsters who used an MP3 player, tablet, game console or TV in the final hour before bedtime.
The researchers, led by Mari Hysing at a regional centre for child health in the city of Bergen, point to several possible explanations.
One is quite simple: that teenagers are getting to bed later and screen time eats into sleep time.
Another idea is that the bright light from devices interferes with circadian rhythm, the day-night system that tells our brain when we should sleep and when we should wake up.
There could also be muscle pains, tension or headaches, for instance from playing a game for too long.
The content consumed may also cause "increased psychophysiological arousal", which means the mind is spinning just as it should be slowing down for the night.
"The recommendations for healthy media use given to parents and adolescents need updating, and age-specific guidelines regarding the quantity and timing of electronic media use should be developed," the study's report states.
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