"Limiting children's screen time linked to better cognition," reports BBC News.
A study of 4,524 children in the US found those who used screens recreationally for less than 2 hours a day did better on tests of mental functioning.
The study was designed to assess whether Canadian recommendations on screen time, sleep and physical activity for children aged 8 to 11 were linked to better mental function, which was assessed using a series of tests.
The recommendations are:
The children who did best on testing were those who followed all 3 recommendations.
But only 5% of children met all 3 recommendations, which could reduce the strength of the association.
And we can't be sure that meeting the recommendations was the cause of the improved test performance.
Screen time and sleep accounted for around 22% of the variation between test results, while physical activity alone didn't seem to be linked to mental functioning.
Other differences, such as children's school grade and ethnic background, were also strongly linked to test results.
The researchers say parents should consider limiting screen time and ensuring adequate regular sleep for children, as well as encouraging physical activity.
UK guidelines on screen time and sleep for children are expected to be published in 2019.
The study was carried out by researchers from the Children's Hospital of Eastern Ontario Research Institute, the University of Ottawa and Carleton University, all in Canada.
It was funded by the US National Institutes of Health.
The study was published in the peer-reviewed journal The Lancet Child and Adolescent Health.
UK media reports were reasonably balanced. Most included warnings that the observational nature of the study means we can't be sure that screen time is directly linked to cognitive function.
The Sun and The Times ignored this by both reporting that limiting screen time "boosts brains".
This was a cross-sectional observational study. These types of studies are fine when researchers are looking for links between factors (such as screen time and mental function).
But cross-sectional studies can't show that one thing causes another. That's because they look at just one point in time, so can't account for changes in brain function or variability in factors like screen time. Also, other factors could influence the results.
Researchers used baseline data from a study of US children that began in 2016.
Children from 21 study sites across the US were invited to take part in cognitive tests.
Children and parents also filled out a series of questionnaires about the child's lifestyle.
For this study, researchers looked at answers to questions about:
Researchers adjusted their figures to take account of some potential confounding factors known to affect cognitive test results:
Researchers found only 5% of children studied met all 3 recommendations.
Children did an hour of physical activity on average 3.7 days a week, used screens on average 3.6 hours a day and slept for an average 9.1 hours a night.
Just over half of children met the sleep recommendations, while 37% met the screen time recommendation and only 18% met the physical activity recommendations.
Children who met all 3 recommendations scored highest on the cognitive tests.
These higher test results seemed most strongly associated with meeting screen time recommendations alone, or a combination of screen time and sleep recommendations.
Meeting the physical activity recommendations alone didn't seem to be linked to cognition test performance.
The researchers said: "These findings highlight the importance of limiting recreational screen time and encouraging healthy sleep to improve cognition in children."
They say doctors, parents, teachers and policymakers "should promote limiting recreational screen time and prioritising healthy sleep routines throughout childhood and adolescence".
The suggestion that children should have limited screen time, enough sleep and plenty of physical activity isn't particularly controversial.
This study adds to evidence that these might be sensible lifestyle adaptations for children.
But this type of study can't prove that any one of these is directly responsible for children's mental abilities.
The study has other limitations. These include:
It's interesting that the study found the strongest link with sleep and screen time combined.
It's possible that overuse of devices like mobile phones at night could be affecting children's sleep, rather than the screen time directly affecting mental functioning.
What's perhaps more interesting is how few children met all the recommendations.
Even the recommendation that children aged 8 to 11 should have 9 to 11 hours sleep a night was only met by 51% of children, while only 18% of children met the recommendation of an hour a day of physical activity.
While the study doesn't give us definitive answers about the effects of screen time, it does provide further evidence to suggest that adequate sleep and limited screen time may improve mental function.
Similarly, frequent physical activity improves physical and mental health.