[1]Medical research is leading American education officials to consider having high schoo

[1] Medical research is leading American education officials to consider having high school classes start later in the morning. The research says that teenagers are more awake later at night than adults are. When staying up late at night, the teenagers have problems learning early in the morning.
[2]Researchers in the state of Rhode Island measured the amount of the hormone(荷尔蒙) melatonin(降黑素) in people's mouths at different times of the day. Melatonin causes people to feel sleepy.
[3]They found that melatonin levels rise later at night in teenagers than they do in children and adults. It remains at higher levels in teenagers later in the morning. They said this shows that teenagers have difficulty learning early in the morning. Yet most school systems in the United States begin high school classes at about seven o'clock.
[4]A few school districts have made some changes. In 1996, school officials in Edina, Minnesota, changed the start of classes from seven thirty to eight thirty. Two years later, the nearby city of Minneapolis did the same. Teachers there reported that students were no longer sleepy in class and were happier. Staying in school later in the day did not seem to be a problem for students who had jobs after school either.
[5]Health experts say that teenagers need between eight and nine hours of sleep a night. Students are likely to be late for school, fail to do their homework, fall asleep in class and have trouble taking part in class discussions.
[6]Some people, however, don't support a later start time because they say that it would limit the time for practicing sports after school. However, the Minnesota schools found that this did not hurt school sports competitions.
[7]More American school districts are discussing the possibility of changing high school start times. Researchers and teenagers say that they cannot make the change quickly enough.
1 What is the best title of the passage? (Please answer within 8 words.)

2 Please fill in the blank in the fifth paragraph with proper words or phrases to complete the sentence. (Please answer within 8 words.)

3 Complete the following statement with proper words.(Please answer within 5 words)
If they go to bed very late at night, teenagers early in the morning.

4 Do you think the change is necessary? Why? (Please answer within 15 words.)

5 What does the word “this”(line 3, paragraph 6) probably refer to?（no more than 4words）

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