A major research project into the effects of heavy screen time among children has found signs of damage to cognitive abilities and memory function.
The $300 million research project being spearheaded by the National Institute of Health in the US also found that children who spend hours each day looking at screens are putting their language skills at risk.
Researchers at the NIH scanned the brains of 4,500 children and found a premature thinning of the brain cortex in those with seven hours a day or more of screen time.
Even children who spent at least two hours a day looking at screens were found to score lower on thinking and language tests.
The ongoing study is examining the impact of technology on children aged nine to 10 over the course of a decade.
Dr Gaya Dowling, an NIH doctor working on the project, said it was too early to draw firm conclusions around the impact of modern technology use by children.
'We don't know if it's being caused by the screen time. We don't know if it's a bad thing,' said Dr Dowling.
'What we can say is that this is what the brains look like of kids who spend a lot of time on screens. And it's not just one pattern.
'It won't be until we follow them over time that we will see if there are outcomes that are associated with the differences that we're seeing in this single snapshot.'
Polls in the UK reveal high levels of smartphone use among children. In a new survey of 2,000 adults by the sports retailer Decathlon, smartphones ranked high up on children's 2018 Christmas wishlists, beating off former favourites like bikes and skateboards.
The most requested gift from children in the run-up to Christmas this year was computer games, with electronic gadgets accounting for six of the top 10 items.
The survey findings suggested most children would be spending their Christmas indoors, as six in 10 parents said they had to encourage their children to step away from their screens over the festive period.
According to the study, the average child will spend just an hour and 35 minutes outside each day over the school Christmas break.
The study also found that a third of British children do not own a bike and among those who do, they last rode it at least one month ago.
Jack Shakespeare, head of Ukactive Kids, a not-for-profit group of organisations that campaigns for better health, voiced alarm over the figures.
'Movement has been stripped out of our children's lives, with 'Generation Inactive' fed a staple diet of sofa play and screen time, while being starved of outdoor activities,' he said.
'It is vital for the health and happiness of this generation that we find ways to get children active and social outside of term time – whether a festive family walk or opening up schools to offer activities during the holidays.'
A 2016 poll by Opinium found that British children on average own their first phone at age seven, followed by an iPad at eight, and a smartphone by age 10.
Last year, Harley Street rehab clinic specialist Mandy Saligari warned that smartphone use was highly addictive for children and urged parents to set boundaries.
'I always say to people, when you're giving your kid a tablet or a phone, you're really giving them a bottle of wine or a gram of coke,' she said.
'Are you really going to leave them to knock the whole thing out on their own behind closed doors?
'Why do we pay so much less attention to those things than we do to drugs and alcohol when they work on the same brain impulses?'