听力原文： For many years, officials of the National Institutes of Health have told Americans that they need to get enough sleep to stay healthy and perform. well. In the past, the N-I-H targeted special groups, like drivers, soldiers and astronauts. Now, health officials have begun a campaign to urge children to get enough sleep. The officials say children need at least nine hours o( sleep every night. They say research shows that children who get this much sleep perform. better in school, suffer fewer accidents and are less likely to become too fat. Studies show that lack of sleep causes tiredness and problems with clear thinking. People who do not get enough sleep become angry easily and have trouble controlling their emotions. Among children, problems that result from lack of sleep often are mistaken for more serious disorders. Unlike adults, tired children seem to have endless energy. Some doctors mistakenly identify this as hyperactivity. Experts say many American teenagers are not getting enough sleep. Teenagers stay up later for several reasons, including schoolwork, after school activities and late- night fun. Many high school students in the United States start school very early in the morning. (33)