Canadian researchers looked at the first bits of data from a 10-year-long USA project meant to study how children's brains develop over time, called the Adolescent Brain Cognitive Development study (or more cleverly, the ABCD study).
The researchers wanted to see if the recommendations in Canada's "24-hour Movement Guidelines for Children and Youth" are effective in ensuring good cognitive development in children.
The study was published last week in the journal "The Lancet Child and Adolescent Health".
The researchers examined how meeting recommendations for 9-11 hours of sleep, less than two hours of recreational screen time, and at least an hour of physical activity every day affected children's cognition.
The current study looked at the results from 4524 children from the ages of eight to 11 who took part in the ABCD study from September 2016 to 2017.
The findings showed that almost 30 percent of the children involved in the study didn't meet any of the recommended guidelines, just over 40 percent met one, 25 percent met two, and five percent met all three.
In the study, 50 percent of the children met the recommended hours for sleeping per night, 37 percent met the recommended screen time, and 18 percent met the recommendation for physical activity. And these children, the researchers found, were more likely to score better on their cognitive tests.
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"Each minute spent on screens necessarily displaces a minute from sleep or cognitively challenging activities", Dr. Bustamante wrote in a commentary that accompanied the study. Also, because the questionnaires were used only at the onset of the study, they don't show the effects of these behaviors - or cognitive development - over time.
"Irrespective of our findings", the authors stress, "physical activity remains the most important behaviour for physical health outcomes, and there is no indication in the literature that it negatively affects cognition".
A groundbreaking study from the National Institutes of Health has found that limiting screen time and increasing physical activity and sleep in children actually improves brain function.
But some studies have suggested that the potential for damage is more about the type of content on these screens and whether parents are involved than about the amount of time using them. The data is also self-reported and could be subject to bias.
For screen time, parents should consider limiting smartphone, TV, or computer use to no more than two hours per day.
"Each minute spent on screens necessarily displaces a minute from sleep or cognitively challenging activities", writes Eduardo Esteban Bustamante, a behavioral scientist at the University of IL, in a commentary that accompanies the study."In the case of evening screen use, this displacement may also be compounded by impairment of sleep quality".
Screen time wasn't the only possible reason for children's worse thinking skills.