参考答案：C

解析：[听力原文] 1-5
M: Teenagers, when allowed to, sleep nearly nine and a half hours every night—as much as young children. But unlike young children, even when teens do get their full sleep, they still have waves of sleepiness in the daytime, and then surges of energy in the evening, making them wide awake late at night. But not for the reasons most of us assume.
F: We kind of always thought that adolescents stayed up late because they liked to, and because there’s plenty of things to do ...
M: But there’s also a big push from biology that makes teenagers such night owls. It comes from that mighty sleep hormone, melatonin.
F: Melatonin is a wonderfully simple signal that turns on in the evening, you’re getting sleepy, and it turns off in the morning.
M: And you awaken. During adolescence, melatonin isn’t secreted until around 11:00 P. M., several hours later than it is in childhood. So the typical teenager doesn’t even get sleepy until that melatonin surge signals the brain that it’s night, no matter how early the teen goes to bed. And the melatonin doesn’t shut off until nine hours later, around 8:00 A.M. But of course most high schools start around 7:30. The result is all too evident. A teenager’s body may be in the classroom, but his brain is still asleep on the pillow.
F: One student says. I’ll wake up and I’ll just feel miserable.
M: An adolescent, and particularly the adolescent in high school, is almost bound to get severely sleep deprived.
F: I know a scientist, that’s William Dement of Stanford University. Bill Dement is Dr. Sleep, captivated by the mysteries of sleep for decades, creating the specialty of sleep medicine.
M: He’s been accepting every invitation that he gets to speak to high school students. So he goes to a high school and it’ll be 10:30 in the morning, or 2:00 in the afternoon, whenever it is, several hundred students in an auditorium, and he’ll just watch them, as he’s talking.
F: Doing a little spontaneous field research.
M: And after ten minutes of sitting, particularly if the lights are dim, almost without exception, they are all struggling to stay awake. Ten minutes!
F: This shows up in lab studies too. The typical teenager when monitored in a quiet environment during morning hours will fall asleep in less than three and a half minutes.
M: It’s just like magic. It’s like somebody turned on some kind of gas ... in the auditorium. And they all look gassed.
F: Not gassed, just severely sleep deprived. Short about two hours of sleep every school night, accumulation into what Dement calls "sleep debt". An estimated 85 percent of high school students are chronically sleep deprived, unable to stay fully awake throughout the school day. And it’s not just falling asleep in class; it’s also riding a bike, playing sports, using tools, driving ... A high school student hit a tree one night when he was driving.
M: Is it true
F: Yes. He told me he fell asleep for couple of seconds, and next thing he knew, he hit a tree.
M: You can have a second where your eyelid blinks and you are not taking information or making judgment.
F: That occurs when you’re at the wheel, you travel 60 feet in that second. The report was that if he would have hit three inches to the left, he would have probably been dead. You know, three inches could have changed everything.
M: Reaction time, alertness, concentration, all slowed down by insufficient sleep. The Federal Department of Transportation estimates teenage drivers cause more than half of all fall-asleep crashes.

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