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  3. Late-Night Drinking Coffee lovers beware. Having a quick" pick-me-up" cup of coffee late in the day will play havoc with you

Late-Night Drinking Coffee lovers beware. Having a quick" pick-me-up" cup of coffee late in the day will play havoc with you

admin2015-07-05  47
问题                         Late-Night Drinking
    Coffee lovers beware. Having a quick" pick-me-up" cup of coffee late in the day will play havoc with your sleep. As well as being a stimulant, caffeine interrupts the flow of melatonin, the brain hormone that sends people into a sleep.
    Melatonin levels normally start to rise about two hours before bedtime. Levels then peak between 2 am and 4 am, before falling again. "It’s the neurohormone that controls our sleep tells our body when to sleep and when to wake, " says Maurice Ohayon of the Stanford Sleep Epidemiology Research Center at Stanford University in California. But researchers in Israel have found that caf-feinated coffee halves the body’s levels of this sleep hormone.
    Lotan Shilo and a team at the Sapir Medical Center in Tel Aviv University found that six volunteers slept less well after a cup of caffeinated coffee than after drinking the same amount of decaf. On average, subjects slept 336 minutes per night after drinking caffeinated coffee, compared with 415 minutes after decaf. They also took half an hour to drop off—twice as long as usual—and jigged around in bed twice as much.
    In the second phase of the experiment, the researchers woke the volunteers every three hours and asked them to give a urine sample. Shilo measured concentrations of a breakdown product of melatonin. The results suggest that melatonin concentrations in caffeine drinkers were half those in decaf drinkers. In a paper accepted for publication in Sleep Medicine, the researchers suggest that caffeine blocks production of the enzyme that drives melatonin production.
    Because it can take many hours to eliminate caffeine from the body, Ohayon recommends that coffee lovers switch to decaf after lunch.
What does the experiment mentioned in paragraph 4 prove?
选项 A、There are more enzymes in decaf drinkers’ urine sample.
B、There are more melatonin concentrations in caffeine drinkers’ urine sample.
C、Decaf drinkers produce less melatonin.
D、Caffeine drinkers produce less sleep hormone that decaf drinkers.
答案D
解析 第四段是第三段实验的继续，是该实验的第二阶段。研究人员通过对志愿者尿样进行化验发现，咖啡因摄入者体内的褪黑激素仅为非咖啡因摄入者的一半。褪黑激素就是睡眠激素，所以答案应该选D。
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