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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 Steep 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 Shito 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.

36.The author mentions “pick-me-up” to indicate that

A.melatonin levels need to be raised.

B.coffee is a stimulant.

C.neurohormone can wake us up.

D.caffeine can balance our brain hormone.

37.Which of the following tells us how caffeine affects sleep?

A.It blocks production of the enzyme that stops melatonin production.

B.It interrupts the flow of the hormone that prevents people from sleeping

C.It halves the body's levels of sleep hormone.

D.It stays in the body for many hours.

38.What does Paragraph 3 mainly discuss?

A.Different effects of caffeinated coffee and decaf on steep.

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 fall asleep.

39.The experiment mentioned in Paragraph 4 finds that caffeine drinkers

A.produce less melatonin.

B.sleep longer than decaf drinkers.

C.produce more urine at night

D.wake up every three hours.

40.Ohayon advises coffee lovers

A.to drink less coffee during the day.

B.to take decaf after lunch.

C.not to go to bed after taking coffee.

D.not to drink coffee after supper

参考答案：BCAAD