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    Johnson is a 10-year-old girl from New York City. She likes staying up late to watch movies on her iPad. Sometimes because she does not have enough sleep, she feels tired at school next morning.

    Just like Johnson, many students feel tired at school in the morning. The kids aged between 6 and 13 need nine to eleven hours of sleep each night, but most of them get less than that.

    What takes the kids' sleep time away? Smart phones and tables (智能手机和平板电脑)! A scientist said, "The kids who have these things in their bedroom often get about half an hour less sleep than those who don't."

    Scientists think a good night's rest is important to students. "The students who sleep enough can learn much better at school than those who don't," another scientist said.

    Johnson's mother said her family decided to make some new rules for her daughter. "we are going to limit(限制) Johnson' screen time and ask her to go to bed earlier than before. We hope these rules can help Johnson to sleep well for school," she said.