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 it's 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 body's 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 don't 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 body's 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 it's day or night.
詞匯：
circadian/s3:'keidi?n/ adj. 晝夜節(jié)奏的，生理節(jié)奏的
adolescent/?d?u'les?nt/ n.青少年;adj.青少年的
puberty/ 'pju:b?ti/ n.發(fā)育;青春期
sync/si?k/ n. (口語)同步;和諧，協(xié)調(diào)
synchronize/'si?kr?naiz / V.(使)同時發(fā)生;(使)同步
注釋：
1.make light of ：輕視，不在乎。例如： We should not make light of their achievements.我們不應(yīng)當(dāng)?shù)凸浪麄兊某删汀?/P>
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 (生物鐘)。
3. stay up：不睡覺，熬夜
4. This shift：這種調(diào)整。指上文所描述的由于生理時間的變化青少年上床時間越來越晚的現(xiàn)象。
5. get your body's clock out of sync with the cycle of light and dark ：打亂了你的生物鐘與晝夜時間循環(huán)之間的平衡
6. gray cloud ：提不起精神的狀態(tài)
7. Brown University in Providence， RI：位于美國羅得島州普羅維登斯的布朗大學(xué)。RI是Rhode Island(羅得島)的首字母縮寫;Providence 是羅得島州的首府。布朗大學(xué)是美國一流大學(xué)，創(chuàng)建于1764 年，是世界聞名的美國“常春藤聯(lián)盟”(還包括哈佛大學(xué)、耶魯大學(xué)、普林斯頓大學(xué)、布朗大學(xué)、哥倫比亞大學(xué)、賓夕法尼亞大學(xué)、達(dá)特茅斯大學(xué)和康奈爾大學(xué))中的一員。
8. the signals that synchronize the body's clock：平衡生物鐘的光信號
練習(xí)：
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 children's 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 eye's 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.
答案與題解：
1.B 第一段第二句提供了答案。句中的your internal clock 即指a clock located inside our Brains。
2. C 第二段主要內(nèi)容是告訴讀者，過了青春期(puberty)，由于生物鐘節(jié)奏的變化，青少年(adolescents and teenagers)比以前要晚睡幾小時。所以C是該段所隱含的內(nèi)容。
3. B 第三段的最后一句直接給出了答案。
4. C 根據(jù)第四和第五段的內(nèi)容，鬧鐘和生物鐘都可以重新設(shè)定時間，但生物鐘能通過眼睛接受的日光來自動調(diào)節(jié)生理節(jié)奏。所以A、B和D都是這兩段中所述內(nèi)容，C是正確選擇，因?yàn)轸[鐘不能自動重新設(shè)定時間。
5.B 問題使用的是過去時，問的是研究者在最新發(fā)現(xiàn)(recent discoveries)之前對眼睛感光系統(tǒng)的認(rèn)識，即，the signals that synchronize the body's clock were handled through the same pathways that we use to see (眼睛所接受到的平衡生物鐘的光信號同樣作用于人類的視覺系統(tǒng))，也就是說，研究者之前認(rèn)為人類的眼睛只有一個感光系統(tǒng)。但最新發(fā)現(xiàn)卻是，人的眼睛有兩個感光系統(tǒng)。
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