Too many teenagers are damaging their health by not getting enough sleep and by falling asleep with electrical gadgets on, researchers say.
Experts say gadgets are distracting at bed time
A third of 12 to 16-year-olds asked slept for between four to seven hours a night. Experts recommend eight hours.
The Sleep Council, which conducted the poll of 1,000 teenagers, says gadgets in bedrooms such as computers and TVs are fuelling poor quality "junk sleep".
Youngsters need to be taught that sleep is important for their health, it said.
Almost a quarter of the teens surveyed admitted they fell asleep watching TV, listening to music or with other equipment still running, more than once a week.
Nearly all had either a phone, music system or TV in their bedroom, and two thirds had all three.
Among 12 to 14-year-old boys, nearly three in five (58%) had a phone, music player, TV and games console in their bedroom.
While 40% of the teens said they were often tired during the day, just 10% placed much importance on getting a good night's sleep.
Dr Chris Idzikowski of the Edinburgh Sleep Centre said: "This is an incredibly worrying trend.
"What we are seeing is the emergence of 'Junk Sleep' - that is sleep that is of neither the length nor quality that it should be in order to feed the brain with the rest it needs.
"Youngsters need to be taught a healthy lifestyle includes healthy sleep as well as healthy food. The message is simple: switch off the gadgets and get more sleep."
Sleep is important for both physical and mental functioning and wellbeing.
Previous studies suggest that people who do not get enough sleep are more likely to be overweight or obese.
UK scientists found sleep deprivation led to hormonal changes which told the body to eat sugary or starchy food to provide an energy boost.
But sleep problems can be a symptom of many other conditions, from problems with the thyroid gland to depression.
Professor Jim Horne, director of Loughborough's Sleep Research Centre, said advising teenagers to get more sleep was "easier said than done".
"I have two teenage kids, and the advice will just fall on deaf ears," he said.