单项选择题

Late-night Drinking

Coffee lovers beware. Having a quick "pick-me-up" cup of coffee1 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 decal. On average, subjects slept 336 minutes per night after drinking caffeinated coffee, compared with 415 minutes after decal. 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 wolfe 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 decal 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 decal after lunch.
The author mentions "pick-me-up" to indicate that______.