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All we have a clock located inside our brains. Similar to your bedside alarm clock， your internal clock2 runs on a 24-hour cycle. This cycle，called a circadian rhythm，helps control when you wake，when you eat and when you sleep. Somewhere around puberty，something happens in the timing of the biological clock. The clock pushes forward，so adolescents and teenagers are unable to fall asleep as early as they used to. When your mother tells you its time for bed，your body may be pushing you to stay up3 for several hours more. And the light coming from your computer screen or TV could be pushing you to stay up even later. This shift4 is natural for teenagers. But staying up very late and sleeping late can get your bodys clock out of sync with the cycle of light and dark5. It can also make it hard to get out of bed in the morning and may bring other problems，too. Teenagers are put in a kind of a gray cloud6 when they dont get enough sleep，says Mary Carskadon，a sleep researcher at Brown University in Providence，RI7 .It affects their mood and their ability to think and learn. But just like your alarm clock，your internal clock can be reset. In fact，it automatically resets itself every day. How? By using the light it gets through your eyes. Scientists have known for a long time that the light of day and the dark of night play important roles in setting our internal clocks. For years，researchers thought that the signals that synchronize the bodys clock8 were handled through the same pathways that we use to see. But recent discoveries show that the human eye has two separate light-sensing systems. One system allows us to see. The second system tells our body whether its day or night. 练习： 1 .The clock located inside our brains is similar to our bedside alarm clock because A it controls when we wake，when we eat and when we sleep. B it has a cycle of 24 hours. C it is a cycle also called circadian rhythm. D it can alarm any time during 24 hours. 2. What is implied in the second paragraph? A Young childrens biological clock has the same rhythm with that of the teenagers. B People after puberty begin to go to bed earlier due to the change of the biological clock. C Children before puberty tend to fall asleep earlier at night than adolescents. D Teenagers go to bed later than they used to due to the light from the computer screen. 3. In the third paragraph the author wants to tell the reader that A it is natural for teenagers to stay up late and get up late. B staying up late has a bad effect on teenagers ability to think and learn. C during puberty most teenagers experience a kind of gray cloud. D it is hard for teenagers to get out of bed in the morning. 4. Which of the following statements is NOT true according to the fourth and fifth paragraphs? A Our biological clock resets itself automatically. B light gets through our eyes and resets our biological clock. C Our internal clock as well as the alarm clock can be reset automatically. D Our internal clock，like the alarm clock，can be reset. 5. According to the last two paragraphs， what did the previous researchers think about the human eyes light-sensing system? A The human eye had two light-sensing systems. B The human eye had one light-sensing system. C The human eye could sense the light of day more quickly than the dark of night. D The human eye could reset our internal clocks in accordance with the alarm clocks. 答案： 1.B 2. C 3. B 4. C 5.B | 
