A．Caffeine blocks production of the enzyme that stops m……
第二篇
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, and tells our body when to sleep and when to wake," says
Maurice Ohayon of the Stanford Sleep Epiderniology 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 decal On average, subjects slept 336 minutes per
night after drinking caffeinated coffee, compared with 415 minutes after decaf.
They also took haft 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.
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.
