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 a.m. and 4 a.m., before falling again. “It’s the neurohormone 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. 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 off4—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 Medicinc, 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.

1．The author mentions “pick-me-up” to indicate that
  A. melatonin levels need to be raised.
  B. neurohormone can wake us up.
  C. coffee is a stimulant.
  D. decaf is a caffeinated coffee.

2. Which of the following tells us how caffeine affects sleep?
  A. Caffeine blocks production of the enzyme that stops melatonin production.
  B. Caffeine interrupts the flow of the hormone that prevents people from sleeping.
  C. Caffeine halves the body’s levels of sleep hormone.
  D. Caffeine stays in the body for many hours.

3. What does paragraph 3 mainly discuss?
  A. Different effects of caffeinated coffee and decaf on sleep.
  B. Different findings of Lotan Shilo and a team about caffeine.
  C. The fact that the subjects slept 415 minutes per night after drinking decaf.
  D. The evidence that the subjects took half an hour to fall asleep.

4.What does the experiment mentioned in paragraph 4 prove?
  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.

5. The author of this passage probably agrees that
  A. coffee lovers sleep less than those who do not drink coffee.
  B. we should not drink coffee after supper.
  C. people sleep more soundly at midnight than at 3 a.m.
  D. if we feel sleepy at night, we should go to bed immediately.