Limiting the amount of time children spend on screens to less than two hours per day is linked with better cognition, the results of a new study indicate.

According to the findings, getting enough sleep is also key.

Cognition refers to how we learn and understand things. Processes involved include thinking, remembering, judging and problem solving.

Researchers in Canada decided to look further into the impact of screen time on cognition in over 4,500 children aged between eight and 11 years.

The children and their parents completed questionnaires and different measures to estimate physical activity, sleep and screen time. The children also underwent a cognition test, which assessed areas such as attention, language abilities, memory and processing speed.

The researchers measured the children against widely recognised guidelines. These suggest that children of this age should be spending no more than two hours per day on screens, they should be getting nine to 11 hours of sleep per night and they should be undertaking at least 60 minutes of physical activity every day.

The study found that 29% of the participants met none of these guidelines, while 41% met only one. Some 25% met two and just 5% met all three recommendations.

Overall, children who spent less time on screens, exercised and got enough sleep had better cognition. Furthermore, the more recommendations the children met, the better their cognitive abilities.

However, meeting just the screen time recommendation, or both the screen time and sleep recommendation, had the strongest links with cognitive development.

While there is a lot of evidence linking physical activity and cognitive development, in this study, meeting the physical activity recommendation alone showed no association with cognition. The researchers said that they were surprised with this finding, but they suggested that the measure used may not have been specific enough.

"We found that more than two hours of recreational screen time in children was associated with poorer cognitive development. More research into the links between screen time and cognition is now needed, including studying the effect of different types of screen time, whether content is educational or entertainment, and whether it requires focus or involves multitasking.

"Based on our findings, paediatricians, parents, educators, and policymakers should promote limiting recreational screen time and prioritising healthy sleep routines throughout childhood and adolescence,"commented Dr Jeremy Walsh of the CHEO Research Institute in Ottawa.

Details of these findings are published in the journal, The Lancet Child & Adolescent Health.