Children lose ground in school because they are losing sleep - and television may be to blame.
Almost seven in 10 children have a television in their bedroom and their parents are mostly not concerned about what they watch, a TES survey has found. Only a quarter use a parental control filter.
There is more concern about what children might get up to on the internet.
More than half of parents (55 per cent) use software to block certain sites.
More than one in 10 children have internet access in the bedroom, and these children are more likely to come from middle-class families. Working-class children and older teenagers are the most likely to have their own television, and 46 per cent of five to seven-year-olds have one.
Half of all children with a television in the bedroom are allowed to watch it between going to bed and going to sleep. As a result, one in five parents said their child had lost sleep during the past month, with older teenagers worrying about schoolwork or exams.
Jim Horne, who runs the sleep research centre at Loughborough University, said healthy five-year-olds need 10 to 12 hours. Eight-year-olds need nine to 11 hours and early teens eight to 10 hours.
Professor Horne suggests banning electronic fun late in the day for younger children with agreed switch-off times for teenagers, but says that reading in bed is fine because they soon fall asleep.
* During April, FDS interviewed 500 parents of children attending primary and secondary state and independent schools in England and Wales.