完形填空。 When you are having class in the early morning, it is not unusual to notice your classmate sitting beside you yawning. He 1 that he didn't have a sound sleep the night before. Many 2 don't get adequate sleep because they are burdened with too much homework, which keeps them up at night doing countless exercise. Some teens cannot 3 at night even when they want to so much. Researches show that during your teenage years, the body's biological clock is ___4_ ,telling you to sleep later at night and wake up 5 in the morning. But 6 in the body clock aren't the only reason teens lose sleep. Lots of people have insomnia(失眠). The most common cause of insomnia is 7 . But all sorts of things can lead to insomnia, including 8 discomfort, emotional troubles and even sleeping environment. It's common for everyone to have insomnia occasionally. But if it 9 for a month or more, go and consult a doctor. Most teens have 10 sometimes, which may make you sweat. But, if they are too frequent, a person's sleep pattern can be seriously 11 . The most common cause of them is emotional 12 , such as fear, stress or anxiety. If you have them a lot, you're also supposed to talk to a 13 . Medical specialists encourage teens to make lifestyle changes to help them develop good sleeping 14 .  Most probably you know that a cup of coffee can keep you 15 at night, but did you know that playing video games or watching TV does the same? (     )1. A. yells (     )2. A. adults (     )3. A. study (     )4. A. broken (     )5. A. sooner (     )6. A. changes (     )7. A. happiness (     )8. A. mental (     )9. A. helps (     )10. A. nightmares (     )11. A. improved (     )12. A. issue (     )13. A. doctor (     )14. A. idea (     )15. A. asleep B. proves B. teens B. work B. reset B. earlier B. moves B. joy B. physical B. reports B. dreams B. destroyed B. affair B. desk-mate B. habits B. alive C. claims C. parents C. sleep C. repaired C. quicker C. patterns C. stress C. biological C. stops C. stress C. advanced C. problems C. teacher C. time C. awake D. complains D. teachers D. communicate D. damaged D. later D. model D. homework D. medical D. lasts D. change D. affected D. things D. friend D. environment D. annoyed