Surveys of American teenagers find that about half of them do not get enough sleep on school nights. They get an average of 60 to 90 minutes less than experts say they need. Experts say teens are biologically programmed to go to sleep later and wake up later than other age groups. Yet many schools start classes as early as seven in the morning. As a result，many students go to class feeling like Danny，16 years old. He plays two sports，lacrosse and football. He is an active teen—except in the morning. DANNY：“Getting up in the morning is pretty terrible. I’m just very out of it and tired. And through first and second period I can hardly stay awake.” Michael Breus，a clinical psychologist with a specialty in sleep disorders says：“These aren’t a bunch of lazy kids—although，you know，teenagers can of course be lazy. These are children whose biological rhythms，more times than not，are off.”Teens，he says，need to sleep eight to nine hours or even nine to ten hours a night. He says sleepy teens can experience a form of depression that could have big effects on their general well-being. It can affect not just their ability in the classroom but also on the sports field and on the road because any tired driver is dangerous，especially a teenager with a lack of experience. So what can schools do about sleepy students？The psychologist says one thing they can do is start classes later in the morning. Eric Peterson，head of St.George’s School in the northeastern state of Rhode Island，says，“In the end，schools ought to do what’s the right thing for their students，first and foremost.”And he wanted to see if a 30-minute delay would make a difference. It did. 小题1:According to the passage，which of the following is the chief cause of sleepy teenagers？
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