"Playing with computer games and watching TV is preventing teenagers from getting enough sleep, according to a report.
Almost one in three are surviving on as little as four hours a night and are going to school exhausted, it said.
The survey for the Sleep Council revealed that "junk sleep" is now a major problem for Britain's teens.
A leading expert linked poor quality sleep with weight gain, with teens possibly craving sugary or high fat foods. The trend for staying over at friends' homes and parents who are too relaxed is adding to the problem, he said.
Teenagers themselves also blamed feeling too hot or too cold, worrying and an uncomfortable bed for their lack of sleep.
The study of 1,000 12 to 16-year-olds found that 30% of teenagers were only getting four to seven hours sleep before a school day instead of the recommended eight to nine hours for this age group.
Almost a quarter (23%) said they fell asleep more than once a week while watching the TV or listening to music or with other electrical items running.
Almost all (99%) said they had a phone, music system or TV in their bedroom and two thirds (65%) had all three. Among 12 to 14-year-old boys, 58% had a phone, music system, TV and games console in their bedroom. A fifth (19%) of boys in this age group admitted their quality of sleep was affected by leaving the TV or computer switched on.
However, across all teenagers surveyed, only one in 10 (11%) said they gave much thought to the quality of their sleep. When asked how they thought lack of sleep affected them, they ranked energy levels top followed by mood, school work, hair and skin, and weight.
Dr Chris Idzikowski, from the Edinburgh Sleep Centre, said: "I'm staggered that so few teenagers make the link between getting enough good quality sleep and how they feel during the day. Teenagers need to wake up to the fact that to feel well, perform well and look well, they need to do something about their sleep." " link
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