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  3. Late-night Drinking Coffee lovers, be careful. Having a quick "pick-me-up" cup of coffee late in the day will interrupt your

Late-night Drinking Coffee lovers, be careful. Having a quick "pick-me-up" cup of coffee late in the day will interrupt your

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问题                         Late-night Drinking
    Coffee lovers, be careful. Having a quick "pick-me-up" cup of coffee late in the day will interrupt your sleep. As well as being a stimulant(兴奋剂), caffeine interrupts the flow of melatonin(褪黑激素), 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(神经激素)that controls our sleep and 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 caffeinated 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(脱咖啡因咖啡). 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.
    In the second phase of the experiment, the researchers woke the volunteers every three hours and asked them to give a urine(尿)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(酶)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 paragraph 3 mainly discuss?
选项 A、Different effects of caffeinated coffee and decaf on sleep.
B、Lotan Shilo’s research design on sleep.
C、What the subjects did after drinking decaf at night.
D、Why some subjects took half an hour to all asleep.
答案A
解析 题意：第三段主要讨论什么?含咖啡因的咖啡与无咖啡因的咖啡对睡眠的不同作用。短文第三段第一句提出“希洛和一个在特拉维夫大学中的萨丕尔医疗中心的团队对6名志愿者进行观察，发现他们喝完一杯含咖啡因的咖啡比喝完相同数量的无咖啡因咖啡时睡得要少”，故选A。
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