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It seems intuitive that children鈥檚 schoolwork will dip if they spend too much time gazing at their phones instead of getting to bed on time or getting some exercise. And that鈥檚 broadly what a Canadian study published this week has found. But is it the final word, and should parents be into pulling the plug on their kids?

Researchers in Canada analysed lifestyle data from questionnaires taken by 4520 American children aged 8 to 11. The children also performed a variety of standard cognition tests.

Jeremy Walsh of the Children鈥檚 Hospital of Eastern Ontario Research Institute in Ottowa, Canada and his colleagues evaluated how well the children met various Canadian government guidelines. These聽suggest limiting screen time to 2 hours a day, sleeping for 9 to 11 hours a night and spending at least an hour being physically active.

Screen limits

More than a third, 1655 children, met the guideline for limiting screen time, and their average performance in the cognitive tests was 4.5 per cent higher than that of the 1330 children who met none of the guidelines. The gain was even higher, at 5.15 per cent, for those meeting both the screen-time and sleep recommendations.

So, armed with these new results, should parents be clamping down on screen use? Walsh himself says the results are provisional. 鈥淎ll these results need to be tempered by the fact it was only a snapshot of children at one point in time,鈥� he says. The US study is running for a further 10 years, and so will enable Walsh and others to track whether the children change their behaviour over time.

鈥淲e can鈥檛 be definitive about directionality, that the extra screen time is depleting cognitive performance,鈥� says Eduardo Esteban Bustamante of the University of Illinois in Chicago. 鈥淚t鈥檚 a 10-year longitudinal study, and so something to keep an eye on.鈥�

Another limitation is that the data didn鈥檛 reveal what the children were doing on their screens, which could be educational or trivial. 鈥淭he study is limited by treating all screen time as equal,鈥� says Heather Kirkorian of the University of Wisconsin-Madison. 鈥淭o truly understand the impact of digital media on children, researchers must understand not only how much, but also how, what, where and with whom they鈥檙e watching.鈥�

It is also unclear whether the questionnaires can produce accurate data. 鈥淭he 8 to 11 year old children reported their own screen and physical activity behaviour, and many may have struggled to do this accurately,鈥� says Kirsten Corder of the University of Cambridge, UK. 鈥淒ata like these are likely to have different types of error which can make it harder to be certain about the results,鈥� she says.

However reliable the results鈥攚hich will become stronger with each year鈥檚 new data鈥攔esearchers agreed that parents should try to set some limits on screen time, especially with bedtime approaching. 鈥淪creen time before bed is doubly problematic because it keeps kids up later, and exposure to light impairs sleep quality,鈥� says Bustamante.

Walsh agrees: 鈥淲e show that excessive screen time before bed has a negative impact on sleep, which is important for development of cognition and the brain generally,鈥� he says.