Making Light of1 Sleep
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.
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.
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.
词汇：
adolescent n.青少年;adj.青少年的
sync n. 同步;和谐，协调
注释：
2. your internal clock ：指的是第一句中的a clock located inside our brains ，也即是第二段第一句中的the biological clock 。
4. This shift：这种调整。指上文所描述的由于生理时间的变化青少年上床时间越来越晚的现象。
6. gray cloud ：提不起精神的状态
创建于1764 年，是世界闻名的美国常春藤联盟中的一员。
练习：
A it controls when we wake，when we eat and when we sleep.
C it is a cycle also called circadian rhythm.
2. What is implied in the second paragraph?
B People after puberty begin to go to bed earlier due to the change of the biological clock.
D Teenagers go to bed later than they used to due to the light from the computer screen.
A it is natural for teenagers to stay up late and get up late.
C during puberty most teenagers experience a kind of gray cloud.
4. Which of the following statements is NOT true according to the fourth and fifth paragraphs?
B light gets through our eyes and resets our biological clock.
D Our internal clock，like the alarm clock，can be reset.
A The human eye had two light-sensing systems.
C The human eye could sense the light of day more quickly than the dark of night.
答案与题解：
Brains。
3. B 第三段的最后一句直接给出了答案。
受的日光来自动调节生理节奏。所以A、B和D都是这两段中所述内容，C是正确选择，因为闹钟不能自动重新设定时间。
pathways that we use to see ，也就是说，研究者之前认为人类的眼睛只有一个感光系统。但最新发现却是，人的眼睛有两个感光系统。