Teachers have backed concerns about the negative impact of TV on children.
Children should be playing rather than watching television, says report
John Bangs of the National Union of Teachers says children can arrive in school "tired and exhausted" from watching too much television.
This follows a report by psychologist Dr Aric Sigman listing 15 health problems that previous studies have attributed to excessive TV viewing.
The report warns that television could contribute to childhood obesity, eyesight problems and hormonal changes.
Dr Sigman warns that access to television should be limited - and that toddlers should not watch any at all.
Among the problems cited by the report are obesity, sleep disruption, diabetes and the triggering of autism.
The report also makes a connection between television watching and changes in the hormone melatonin, which is linked to the immune system and the onset of puberty.
Researchers have claimed that excessive television viewing can suppress melatonin and induce puberty at an earlier age.
Too much television makes children too tired for school, warn teachers
"While society has shown alarm over school dinners, it has ignored the high-screen diet children have been consuming," says Dr Sigman.
In particular, Dr Sigman warns about the vulnerability of under-three-year-olds to being over-exposed to television.
And he raises concerns that many young children now have televisions in their bedrooms.
Mr Bangs, head of education at the NUT, says that teachers are already aware of the effects of children watching too much television.
"If you've been watching until two in the morning, you come in tired and exhausted.
"There has been quite a lot of work done in schools to try and shift that by talking to parents and building a structure into the school day, such as breakfast clubs," said Mr Bangs.
But sociologist Professor Frank Furedi casts doubt on the claims - and says children's television viewing was in decline, as they shifted towards computer games and the internet.
"It seems that every week we get another report that tells us about another threat to our children," he said.
"We seem to have lost the capacity to simply say that it's not a good idea for children to watch too much TV - I think that every parent knows that children should be out playing in the open air.
"But instead of saying that, we have to invent all these negative factors about obesity and autism," Professor Furedi said.