We all know instinctively that watching a great deal of television is not good for children. Where experts and parents differ is on the question of how much is too much. The main concerns have invariably been that children who are glued to the screen instead of running about outside are at greater risk of becoming fat, and that some youngsters will be absorbing unsuitable material, particularly post-watershed scenes of sex and violence.

We all know instinctively that watching a great deal of television is not good for children. Where experts and parents differ is on the question of how much is too much. The main concerns have invariably been that children who are glued to the screen instead of running about outside are at greater risk of becoming fat, and that some youngsters will be absorbing unsuitable material, particularly post-watershed scenes of sex and violence.

What will disturb many people about the study by Dr Aric Sigman published today is the finding that watching television triggers a number of biological changes, irrespective of whether we are tuned into a violent shoot-out or the most calming of gardening programmes. One of his findings is that television viewing suppresses the production of melatonin, which has important roles in the immune system, sleep/wake cycle and the onset of puberty. Since we know that children, particularly girls, are reaching puberty at an earlier age than previous generations, and we continually hear complaints from teachers about children who are are tired when they come to school in the morning, it is possible there is a hormonal link.

The report says that watching TV is associated with irregular sleeping patterns in toddlers, and even passive viewing is strongly related to sleep difficulties in five- to six-year-olds. We know from the explosion in advice about getting children to sleep that more and more parents of young children find that implementing a regular bedtime is a losing battle rather than a peaceful ritual of a story and a cuddle.

That story may provide a clue: according to neuroscientists, watching television is a "non-intellectually stimulating activity" for brain development, which is not the case for reading. It may be that children are not tired because not only have their bodies not had enough exercise, but neither have their brains.

The increase in type 2 diabetes has been attributed to dietary changes, but there is now a suggestion that sitting watching television could be the reason for more people suffering from abnormal glucose metabolism. These are just some of 15 biological effects Dr Sigman attributes to television viewing, which also include serious medical risk factors such as changes in skin immune cells, elevated serum cholesterol, a decrease in metabolic rate, permanent eyesight damage and an increased likelihood of eventually developing Alzheimer's disease. Taken together, they are a frightening prospect, and it is tempting to dismiss them as alarmist, but the fact that both the number of hours spent watching television and the age at which a child begins viewing are related to the later negative effects must make us consider them seriously.

In Britain we have taken a particular pride in the quality of the television programmes we make. That may be one reason why the British spend more time watching TV and less time reading than any other nation in Europe. Since these findings appear just days after the wellbeing of British children was ranked among the lowest in 21 industrialised countries, we have nothing to lose by reaching for the off switch.