Internal clocks keep pace with the 24-hour cycle of day and night and light and dark, enabling organisms to adapt their behavior accordingly. Whether these daily—or circadian—rhythms(节奏) help a single-celled organism expect the first rays of morning light or a mouse prepare for a busy night searching for food, they are a universal feature of life on earth.
　　Circadian clocks govern many human activities. From sleep-starved students to active old people, the body automatically observes a daily regulation of wakefulness and sleep. Our internal clocks keep us mentally watchful during the day and prompt us to rest and recover during the night. The pattern continues even in the absence of external signals from the rising and setting sun.
　　Research volunteers living in a dark room naturally maintain a roughly 24-hour cycle of activity and rest. Researchers have concluded that sunrise can reset the clock but that the clock does not depend on sunrise and sunset to keep time. We generally take our biological clock for granted, but as anyone who has experienced the misery of jet lag (时差反应) or worked the night shift knows, we will suffer if our natural rhythms are disturbed. In rare cases, severe disturbances in the body clock can be inherited, with problems affecting members of particular families. Individuals suffering from earlier sleep, for example, often fall asleep in the late afternoon or early evening, only to wake up in the middle of the night.
　　Despite a long-standing interest in biological clocks, only in recent years have scientists made major progress in understanding the molecular systems that regulate them. The circadian pacemaker—control center—in humans is located in the brain. A cluster (簇) of only several thousand nerve cells governs a wide range of 24-hour variations in our body, ranging from changes in hormonal levels and body temperature to exposition to disease. Understanding the detailed workings of the circadian clock may explain why heart attacks occur more often in the morning and why the incidence of coughing is more common at night.
　　1. The main idea of this passage can be any one of the following EXCEPT ________.
　　A. internal clocks
　　B. circadian clocks
　　C. mental clocks
　　D. biological clocks
　　2. The daily changes in our body are governed by ________.
　　A. scientists
　　B. humans ourselves
　　C. our mind
　　D. nerve cells
　　3. What does the case of “research volunteers living in a dark room” indicate?
　　A. The dailyrhythms continue despite outside influences.
　　B. Those volunteers continued their cycle without any changes.
　　C. The pattern remains unchanged with the internal influences.
　　D. Sunrise and sunset help the volunteers to keep time.
　　4. How do internal clocks affect our health?
　　A. Many of us have the miserable experience of jet lag.
　　B. The night shift is among those we can not avoid.
　　C. We will not feel well if natural rhythms are changed.
　　D. Problems with internal clocks can usually be solved within a family.
　　5. It can be inferred that ______________.
　　A. we now know why certain health problems occur more often at particular times.
　　B. it is now easy to find out one's health problems.
　　C. we don't know the causes of heart attacks at times.
　　D. coughing problem is not serious at daytime. 
　　参考答案：1. C    2. D    3. A    4. C    5. A 
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