仔细阅读下列短文, 根据题目, 选出最佳选项. It is reported that many middle school students are just not getting enough sleep, especially during the school week. The problem seems to get worse as they get older. Year 7 students sleep 8.4 hours in a school night, and Year 9 students only 6.9 hours. It's also reported that not getting enough sleep can cause problems in a student's life. Many students fall asleep in school or while doing their homework, so it is not surprising that they get lower grades than those who get enough sleep. Scientists suggest nine hours a night for middle school students. Of the students who feel unhappy and nervous, 73％ don't get enough sleep at night. Why aren't students getting enough sleep Many students have one of the following things, like telephone, television, or computer in their bedrooms. Using them on a school night not only takes their time away from homework, but also makes it difficult for them to fall asleep. As a result, many students can't fall asleep before 11 p. m. , yet they must get up early to go to school. Scientists also suggest students should not do exciting activities for an hour before bedtime.

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