A study of 577,000 children from 42 countries shows girls get ill after just two hours watching screens like television, chatting or gaming online. But because boys are more likely to be physically active, they can safely spend twice as much time on screens each day before having health problems.

Doctors warn (告诫) that depression (抑郁) is rising among Australian children. The study also shows the need to balance (平衡) screen time with "green time" outdoors. Children who exercise or play outdoors are healthier and happier, the study shows.

Professor Asaduzzaman Khan said the study found boys were twice as likely as girls to meet the health guideline for physical activity of an hour a day. He said 24 percent of boys, while 14 percent of girls, spent an hour a day exercising.

A dad from Melboume. Tom Hindhauhh said he had a three-zone system for his thirteen-year-old son, Patrick: a "go zone" for time to do schoolwork, a "slow zone" for time to play video games or chat with friends on line and a "no zone" when screens were turned off.

"We need those screens for homeschooling. Technology is also the only way kids can communicate and hang out with each other during the COVID-19," he said. "We have an hour or so for slow zone and after that screens should be off. We need to get children outside and moving. Physical activity is the key for their health, whether it's walking, riding a bike or playing balls.

Professor Khan said spending too much time on watching TV, gaming or chatting online could lead to depression, overweight, unhealthy diet and decreased cognitive abilities (认知能力减弱)." If you spend more time on screens, you are likely not to do well in your studies."