Small children who spend hours staring at the screens every day are more badly kept until they're five.
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.
Time on the screen has a "significant impact" on the development of the child, the researchers say, warning that parents must limit it.
One of the authors of the study suggested that this is because the time spent watching screens is the time taken away from healthier activities such as sports or sleep.
According to the researchers, only half an hour a day or even less would be the optimal amount for pre-school children.
However, experts in this field immediately dismissed these findings as "critical shortcomings" and doing nothing to prove that screen time has actually caused bad behavior.
The researchers who conducted the study say that young children should not spend more than 30 minutes a day looking at phones, tablets, computers or televisions (photo backup)
Researchers at the University of Alberta have studied over 2,400 families and found that children glued to the screens have greater behavioral problems.
In addition to the greater risk of Attention Deficit Hyperactivity Disorder (ADHD), those exceeding two hours a day were five times more inattentive.
"We found that screening time had a significant impact at the age of five," said Dr. Piush Mandhane.
Three-year-old children spent an average of an hour and a half on screens every day. It fell slightly to 1.4 hours for five-year-olds.
Researchers have found that screening time can have even more impact on a child's behavior than how much sleep they have obtained or how stressed their parents are.
This may be due to the fact that it differs from other aspects of life, which may reduce the risk of attention problems.
Dr. Mandhane told MailOnline: "Our data suggests that more time on the screen leads to a shorter sleep time.
"Developing a regular routine of sleep, consistent waking and bedtime, which limits sleep time, as well as an important part of growth, development and behavior.
"In another analysis, we found that children who watched more than 2 hours of time per screen per day were nearly 65 percent less likely to sleep 10 hours a day. So more time on the screen means less time to sleep. "
The study confirms previous studies also suggesting sleep damage and other studies indicating worse brain development, mental health problems and damaged eyes.
Lack of sleep in childhood can inhibit brain growth and thus lead to problems later in life.
HOW MANY TIMES ON THE SCREENS HAVE INFLUENCE ON CHILDREN?
Research has shown that spending too much time browsing screens – for example, smartphones, tablets, computers and TVs – can be harmful to children's intelligence, sleep, mental health and sight.
A study by the CHEO Research Institute in Ottawa in 2018 has shown that eight to eleven-year-olds have five percent worse results in brain power tests than their peers if they spend two hours a day on screens.
They suggested that this may be due to the fact that looking at screens is not as stimulating as reading and can interfere with a good night's sleep.
The disturbed sleep was also the subject of a warning by the British Royal College of Pediatrics and Child Health earlier this year, when he advised children not to use screens before going to sleep.
RCPCH said that high screening time is associated with a less healthy diet, sedentary lifestyle and worse mental health.
Dr. Max Davie, a health specialist, said: "Parents need to be in control of their time on the screen if they want to control the time spent on the screen by the family. It is much easier to be authoritative if you practice what you preach. "
Dr Langis Michaud, professor of optometry at the University of Montreal, wrote in The Conversation in February: "Since the introduction of the smartphone in 2007, there has been a dramatic increase in visual problems.
"Although the device itself does not emit harmful radiation, it requires the user to read its screen at a distance of 20 cm, not a normal distance of 45 cm to 50 cm.
"It has been suggested that this close proximity increases the risk of myopia by eight times, especially if both parents are short-sighted."
Researchers at the CHEO Research Institute in Ottawa found that children between the ages of 8 and 11 have a 5 percent worse brain function than their peers if they spend more than two hours a day looking at the screen.
They think that this may be due to the fact that games and movies do not stimulate the brain in the same way as, for example, reading a book.
Probably it meant that they did not sleep as well as others.
In a study published today, researchers found that organized sport and good sleeping can actually protect brains from the bad effects of excessive screen time.
They found that exercise alone is less important for improving children's behavior than participating in organized activities.
Dr Tamana added: "The more children spend playing sports, the less likely they are to have behavioral problems.
"Many things that you do through organized activities are really important for young children at an early stage.
"I think instead of screen time it would be beneficial for parents to increase their chances for other structured activities."
While scientists suggested that "less is more", they did not recommend a total cut-off.
Instead, they said it was a good opportunity to make sure that children use their electronic devices wisely.
"Our data suggests that from zero to 30 minutes a day is the optimal amount of time on the screen," said Professor Mandhane.
"Pre-school period is the perfect time to learn healthy screen relationships."
Researchers in the UK criticized the study and found that it does not directly connect screen time with bad behavior or ADHD.
They added that the researchers crossed the border in giving advice to parents and doctors on the basis of defective paper.
Professor Andrew Przybylski, Research Director at the University of Oxford's Institute of Internet Research, said: "There is no data on the behavior of children, so it is possible that children who are predisposed to behavioral problems are also predisposed to a higher screen level. Paper does not give this context.
He added: "The authors go far beyond their results in providing advice to physicians and educators. Correlations are very small and inconsistent.
"It is quite shocking that the authors will promote screen time limitation based on these findings, taking into account that the evidence presented in the article suggests that almost every other analyzed factor was a much stronger predictor".
Dr. Bob Patton, lecturer in clinical psychology at the University of Surrey, said: "Although [the study] suggests that children under five who spend an average of two hours or more a day in front of screens are more likely to have an ADHD diagnosis, which gives no indication that screen time has caused problems.
"While the abuse of an" electronic babysitter "may contribute to the development of behavioral problems, parents should remember the possibilities and ensure that young children participate in various activities, both on and off the screen."
The research was published in the journal PLOS ONE.