American researchers found that the metabolic rate of children was significantly lower while watching television than while lying on a couch and resting.
They warn that children who watch excessive television are more likely to be fat, because their energy expenditure while watching is lower than if they were doing nothing at all.
Unresponsive and near-comatose children who spend much of their lives slumped in front of a television are common. But parents will be surprised to learn that their offspring may be in a 'deep, deep state of relaxation' while viewing, according to Mary Lee Shelton, a research assistant in the Department of Psychology at Memphis State University.
Ms Shelton and researchers at the University of Tennessee studied 15 obese children and 16 children of normal weight aged 8 to 12 years. Their metabolic rate was measured when they were resting, and when they were watching a popular children's show called The Wonder Years.
Ms Shelton found that watching the programme lowered the metabolic rates of both groups of children, and that this was more noticeable in the obese.
'There are two basic conclusions,' Ms Shelton said. 'One is that television puts the kids into a deep, deep, state of relaxation. The other is that children fidget more while they are resting (which increases the metabolic rate).' The study, published in Pediatrics journal, involved only a small number of children and one television programme, and further research is necessary, she added.
Dr Anne Sheppard, a child psychologist at Leeds University, said: 'Fighting with an older brother would be better for a child's development than watching television.'