Sixty-three percent of children spent more than two hours a day staring at screens, failing to meet the screen-time limit.
The study was conducted by researchers from the CHEO Research Institute's Healthy Active Living and Obesity using data from the U.S. National Institute of Health.
A Canadian research team looked at data from 4,500 United States children ages 8 to 11 and compared the kids' self-reported screen use to their performance on a test that measures markers of brain development.
The findings were published in Lancet Child & Adolescent Health.
Half of the children met the sleep recommendation (51%, 2,303/4,520), 37% (1,655/4,520 children) met the screen time recommendation, and 18% (793/4,520 children) met the physical activity recommendation. Children who spent under two hours on screens scored, on average, about 4 percent higher on thinking-related tests than the kids who didn't meet any of the screen, exercise, or sleep guidelines.
The study associates kids who met the guidelines - which include 9 to 11 hours of sleep, at least one hour of physical activity, and less than two hours on screens - with improvements in cognition.
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"Behaviours and day-to-day activities contribute to brain and cognitive development in children and physical activity, sedentary behaviour, and sleep might independently and collectively affect cognition", he is quoted as saying by the Daily Mail.
Researchers kept in mind other factors that may have influenced the results such as the net household income, race and ethnicity, the body max index and other useful indicators, but they can not claim that the study is completely accurate.
"This new research adds to existing evidence, and supports concerns about screen time and potential negative links with cognitive development in children", Kirsten Corder, a senior investigator scientist with the MRC Epidemiology Unit at the University of Cambridge, said in a statement. While 41% of children hit just one of these guidelines, and 25% hit two, only 5% of the children met all three recommendations.
"The more individual recommendations the child met, the better their cognition", the study concluded, noting that screen time was the most important factor. They were asked how much they sleep, how much they exercise and how much time they spend in front of screens.
A similar pattern was seen with children who both got enough sleep and less screen time, further supporting a link between poor sleep and screens. Research is ongoing, however, with particular focus into what exactly kids are watching with their screen time, be it for entertainment or educational purposes.
"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".