Overall cognition skills were best among the one in 20 children who got between nine to 11 hours sleep, less than two hours recreational screen time, and at least an hour's exercise daily.
These children did around five per cent better in the tests than the average child. Significantly, the study isolated screen time as the likely key factor. Children who were glued to their screens for less than two hours a day saw performance around four per cent better than the average among their group, regardless of other habits.
Lead researcher Dr Jeremy Walsh told the Telegraph: "These are landmark findings. We've set a clear two hour benchmark here and it shows clear cognitive benefit is associated with keeping within that limit.
"I think parents should now be looking closely at screen time and this suggests it should be limited to two hours a day." The findings are from an observational study- meaning it was not possible to prove that extra time on devices caused the weaker cognitive skills. But experts believe several factors- the impact of blue light on the brain, the online activity itself and the fact time spent on gadgets eats into time which might otherwise be spent exercising, sleeping or taking part in more social or mentally challenging tasks, could explain the links.
The findings are likely to be considered by Dame Sally Davies, the country's chief medical officer, who is undertaking a review of the impact of technology on children's health, and whether to set guidance on healthy screen time. Until now, experts have argued about a lack of clear evidence about the impact of screen time.
It comes as the Telegraph's campaign calling for a statutory duty of care on all social media and online gaming companies, opening them up to legal action when standards are breached. Dr Walsh said more research was needed to examine the potential impact of different online activities.
In the meantime he urged parents and health officials to consider two hours recreational time online as the maximum daily limit.
"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," he said.
Writing in a linked Comment, Dr Eduardo Esteban Bustamante, from the University of Illinois, USA, said the findings suggest that too much screen time could mean children's brains had too little time to recover from the strain of each day.
"Each minute spent on screens necessarily displaces a minute from sleep or cognitively challenging activities. In the case of evening screen use, this displacement may also be compounded by impairment of sleep quality," he said.
The study found that US children spend an average of 3.6 hours a day engaged in recreational screen time. The research controlled for household income, parental and child education, ethnicity, pubertal development, body mass index and whether the child had had a traumatic brain injury.
In the study, families were given questionnaires to see how many children aged 8 to 11 get nine to 11 hours sleep, less than two hours recreational screen time, and at least an hour's exercise daily.
The study found 29 per cent of children achieved none of the three recommendations, while 41 per cent met just one, 25 per cent met two and just five per cent met all three.
Just over half of the children met the sleep recommendation, while round one third met the screen time recommendation, and 18 per cent met the physical activity recommendation.
While achieving all three was associated with the best performance in the tests, when individual factors were examined levels of physical activity made little difference, with the strongest association linked to screen time.
First published on The Telegraph.
