A Brown University sleep researcher has some advice for people who run high schools: Don’t start classes so early in the morning. It may not be that the students who nod off at their desks are lazy. And it may not be that their parents have failed to enforce(确保) bedtime. Instead, it may be that biologically(生物学上) these sleepyhead(瞌睡虫)students aren’t used to the early hour.
　　“Maybe these kids are being asked to rise at the wrong time for their bodies,” says Mary Carskadon, a professor looking at problem of adolescent (青春期的)sleep at Brown’s School of Medicine.
Carskadon is trying to understand more about the effects of early school time in adolescents. And, at a more basic level, she and her team are trying to learn more about how the biological changes of adolescence affect sleep needs and patterns(方式).
Carskadon says her work suggests that adolescents may need more sleep than they did at childhood, no less, as commonly thought.
　　Sleep patterns change during adolescence, as any parent of an adolescent can prove. Most adolescents prefer to stay up later at night and sleep later in the morning. But it’s not just a matter of choice---their bodies are going through a change of sleep patterns.
　　All of this makes the transfer(迁移)from middle school to high school---which may start one hour earlier in the morning—all the more difficult, Carskadon says. With their increased need for sleep and their biological clocks set on the “sleep late, rise late” pattern, adolescents are up against difficulties when they try to be up by 5 or 6 a.m. for a 7:30 a.m. first bell. A short sleep on a desktop may be their body’s way of saying. “I need a timeout.”
Carskadon suggests that high schools should not start classes so early in the morning because
_______.

A．it is really tough for parents to enforce bedtime

B．it is biologically difficult for students to rise early

C．students work so late at night that they can’t get up early

D．students are so lazy that they don’t like to go to school early

The underlined phrase nod off most  probably means _______.

A．turn around

B．agree with others

C．fall asleep

D．refuse to work

What might be a reason for the hard transfer from middle school to high school?

A．Adolescents depend more on their parents.

B．Adolescents have to choose their sleep patterns.

C．Adolescents sleep better than they did at childhood.

D．Adolescents need more sleep than they used to.

What is the passage mainly about?

A．Adolescent health care.

B．Problems in adolescent learning.

C．Adolescent sleep difficulties.

D．Changes in adolescent sleep needs and patterns.