Walsh and his team looked at data - based on detailed questionnaires - for 4,520 children spread across 20 locations in the United States.
Kids with the sharpest intellects spent less than two hours a day on their cellphones, tablets and computers, coupled with 9 to 11 hours of sleep and at least an hour of physical activity, the study found.
Nearly a third of the American children are outside all three recommendations, shows the study published on 27 September 2018 in The Lancet Child & Adolescent Health.
The study followed more than 4,500 USA children aged between eight and 11, noting how much of their day involved recreational screen time, physical activity and sleep.
The more individual recommendations the child met, the better was their cognition, the findings showed. Sixty-three percent of children spent more than two hours a day staring at screens, failing to meet the screen-time limit.
Although the report was focused on children in the US, Walsh says the findings could be applicable in any country.
The researchers found that only 5 percent of children met all three recommendations.
Other organizations, such as the American Academy of Pediatrics have guidelines in place to help with the management of children's screen time. He warns that, in that sense, the new study's findings "are a potential canary in a coal mine". In the case of evening screen use, this displacement may also be compounded by impairment of sleep quality.
"The link between sedentary behaviors, like recreational screen time, is unclear as this research is in the early stages and it appears to vary depending on the types of screen-based activity", he said. That's what the researchers found when they compared guideline adherence against performance on brain exercises ("cognition"). While those with excess screen time scored lower on cognitive tests, this does not necessarily mean TVs and smartphones are the direct cause: it could be that more cognitively-capable children are less likely to spend time on screens, Science News reports.