Too many teenagers are damaging their health by not getting enough sleep and by falling asleep with electrical gadgets on, researchers say.

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.