Children and parents completed questionnaires at the start of the trial to estimate the child's physical activity, sleep and screen time.
It found that 51 percent of the children got the recommended nine to 11 hours of uninterrupted sleep per night, 37 percent met the recreational screen time limit of two hours or less per day, while 18 percent met the physical activity recommendation of at least 60 minutes of accumulated physical activity a day.
In addition, meeting only the screen time recommendation or both the screen time and sleep recommendations had the strongest associations with cognitive development.
Only one in 20 kids in the United States meets guidelines on sleep, exercise and screen time, and almost a third are outside recommendations for all three, according to a study published on Thursday.
As part of the study, 4,500 children in the United States were surveyed on the behaviors and were also assessed on their cognitive ability which included language, memory, processing speed, and attention. The authors note this was a surprising finding and may suggest that the measure used may not have been specific enough. 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.
"Based on our findings, pediatricians, parents, educators, and policymakers should promote limiting recreation screen time and prioritizing healthy sleep routines during childhood and adolescence", Jeremy Walsh, the study's lead author, told The Guardian.
"We found that more than two hours of recreational screen time in children was associated with poorer cognitive development".
According to the study's authors, additional research is needed to better understand the effects of different kinds of screen time on cognition, and they point out that given the study's observational nature, it does not prove a causative link between screen time on cognition.
The authors note some limitations, including that their study is observational so can not establish the underlying causes or the direction of the association. In the case of evening screen use, this displacement may also be compounded by impairment of sleep quality.