Toddlers who spend hours staring at screens every day are more badly behaved by the time they are five, a study has claimed.
Pre-school children who use smartphones, tablets and other gadgets for more than two hours a day are also seven times more likely to develop ADHD.
The screen time has a 'significant impact' on the child's development, researchers said as they warned parents need to cut it down.
One author of the study suggested this is because time spent looking at screens is time taken away from healthier activities such as sport or sleep.
Just half an hour per day, or even less, would be the optimum amount for pre-school aged children, according to the researchers.
But experts in the field immediately dismissed the findings as having 'critical shortcomings' and doing nothing to prove the screen time had actually caused the bad behaviour.
The researchers who carried out the study say toddlers shouldn't spend more than 30 minutes per day looking at phones, tablets, computers or TVs (stock image)
Scientists at the University of Alberta studied more than 2,400 families and found children glued to screens have more significant behavioural problems.
As well as a higher risk of attention deficit hyperactivity disorder (ADHD), those exceeding two hours per day were five times more likely to be inattentive.
'We found screen time had a significant impact at five years of age,' said Dr Piush Mandhane.
Three-year-olds in the study spent an hour-and-a-half, on average, looking at screens every day. This fell slightly to 1.4 hours for five-year-olds.
The researchers found screen time may even have a bigger effect on a child's behaviour than how much sleep they get or how stressed their parents are.
And this may be because it takes away from other aspects of life which could reduce the risk of attention problems.
Dr Mandhane told MailOnline: 'Our data suggests that more screen-time leads to less sleep-time.
'Developing a regular sleep routine, consistent wake and bed times that limit screen-time prior to bed, in also an important part of growth, development, and behaviour.
'In another analysis, we found that children who watched more than 2 hours of screen time per day were almost 65 per cent less likely to sleep 10 hours per day. So more screen time equals less sleep time.'
The study backs up past research also suggesting damage to sleep, and other studies pointing to poorer brain development, mental health issues and damaged eyes.
A lack of sleep in childhood could stunt the growth of the brain and therefore lead to problems later in life.
Research has shown spending too much time looking at screens – smartphones, tablets, computers and televisions, for example – can be damaging to children's intelligence, sleep, mental health and vision.
A 2018 study by the CHEO Research Institute in Ottawa found eight to 11-year-olds performed five per cent worse on brain power tests than their peers if they spent two hours per day looking at screens.
This, they suggested, may be because looking at screens isn't as stimulating as reading, and could interfere with vital sleep.
Disturbed sleep was also the focus of a warning from the UK's Royal College of Paediatrics and Child Health earlier this year, when it recommended children don't use screens before bed.
The RCPCH said high levels of screen time are linked to a less healthy
