A、Because it is really tough for parents to enforce bedtime. B、Because it is biologically difficult for students to rise early.

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 bedtime. Instead, it may be biologically that these sleepyhead 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 problems of adolescent sleep at Brown’s School of Medicine.
    Carskadon is trying to understand more about the effects of early school time on 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.
    Carskadon says her work suggests that adolescents may need more sleep than they did at childhood, not 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 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 it comes to trying to be up by 5 or 6 am for a 7:30 am first bell. A short sleep on a desktop may be their body’s way of saying, "I need a timeout."
Questions 16 to 18 are based on the passage you have just heard.
16. Why does Carskadon suggest that high schools should not start classes so early in the morning?
17. What might be a reason for the hard transfer from middle school to high school?
18. What is the passage mainly about?