The researchers suggested parents limit their children's screen time for the sake of their mental and physical health.
A study by the HKU researchers last year which surveyed 7,585 primary school students showed that nearly 75 percent of the polled children spent more than two hours per day on digital devices, compared to 47 percent for children in Beijing and 20 percent in the United States.
In another study, the researchers from HKU's faculty of medicine studied the use of digital devices by 681 children in 2011 when they were five years old, and then followed up with them when they were nine years old.
Their findings showed that for each hour per day spent watching TV or playing video games, the risk of physical and mental problems increased dramatically, local radio RTHK quoted the researchers as saying.
Nine-year-olds were nearly 20 percent more likely to be overweight for every hour they spent watching TV each day at the age of five. Every extra hour of TV also raised the chance of behavioral problems by a staggering 44 percent, while chances of emotional problems rose by 38 percent, the researchers said.
Those who played video games also saw their chances of developing emotional problems rise by 23 percent. The risk of hyperactivity was also higher, they said.
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