Only one in 20 kids in the United States meets guidelines on sleep, exercise and screen time, and nearly a third are outside recommendations for all three, according to a study published Thursday.

On average, children aged eight to 11 spent 3.6 hours per day glued to a TV, mobile phone, tablet or computer screen, nearly double the suggested limit of two hours, researchers found.

Too little sleep and excess screen time were clearly linked to a drop off in cognitive skills, such as language ability, memory, and task completion, they reported in The Lancet Child & Adolescent Health.

"We found that more than two hours of recreational screen time in children was associated with poorer cognitive development," said lead author Jeremy Walsh, a researcher at the Children's Hospital of Eastern Ontario Research Institute.

"Based on our findings, paediatricians, parents, educators and policymakers should promote limiting recreation screen time and prioritising healthy sleep routines during childhood and adolescence."

Walsh and his team looked at data -- based on detailed questionnaires -- for 4,520 children spread across 20 locations in the United States.

They also tested the kids for six kinds of cognitive skills, adjusting the results for household income, puberty development and other factors that might affect performance.

The results were measured against the Canadian 24-hour Movement Guidelines for children.

For sleep and exercise, the recommendations align with those of the World Health Organization, but Canada is the first country to propose limits for time spent in front of a back-lit screen.

- Distraction in the classroom -

Nearly 30 percent of children failed to meet any of the recommendations, more than 40 percent met only one, a quarter met two, and only five percent conformed to all three.

Half the kids were getting enough sleep, 37 percent remained within the screen-time limits, and only 18 percent met the physical activity recommendation.

"The more individual recommendations the child met, the better their cognition," the study concluded, noting that screen time was the most important factor.

In contrast to earlier research, lack of exercise did not correlate with poorer performance on the cognition tests.

The strong link between time spent staring at a screen and brain function "potentially reflect the interruption of the stress-recovery cycle needed for growth in children," commented Eduardo Esteban Bustamante, an assistant professor at the University of Illinois' College of Applied Health Sciences who did not take part in the study.

"Each minute spent on screens necessarily displaces a minute from sleep."

Scientists and educators have grown increasingly concerned that constant use of mobile phones from an early age may lead to problems ranging from addiction to attention deficit disorder.

The vast majority of teachers in a recent survey said that smartphones had become a distraction in the classroom, eroding the ability of students to focus.

France urges parents not to allow children under three to watch TV, and American paediatricians also favour a total ban on screen time until at least 18 months.