Late-Night Drinking

Coffee lovers beware.Having a quick"pick-me-up"cup of coffee late in the day will playhavoc with your sleep.As well as being a stimulant,caffeine interrupts the flowof

melatonin,the brainhormone that sends people into a sleep.

Melatonin levelsnormally start to rise about two hours before bedtime.Levels then peak be-tween2 a.m.and 4 a. m.,before falling again."It's the neurohormone thatcontrols

our sleep and tellsour body when to sleep and when to wake,"says Maurice Ohayon of the Stanford SleepEpidemiology ResearchCenter at Stanford Universityin

California. But researchers in Israel havefound that caffeinated coffee halves the body's levels of this sleep hormone.

Lotan Shilo and ateam at the Sapir Medical Center in Tel Aviv University found that six vol-unteers slept less well after a cup of caffeinated coffee than after drinkingthe

same amount ofde-caf（脱因咖啡）.On average ,subjects slept 336 minutes per night after drinking caffeinatedcof-fee,compared with 415 minutes after decaf. They also

took half an hour todrop off-twice as long as usual-and jigged around in bed twice as much.

In the second phaseof the experiment,the researchers woke the volunteers every three hours andasked them to give a urine sample,Shilo measured concentrations of a

breakdown product ofmelatonin .The results suggest that melatonin concentrations in caffeinedrinkers were half those in decaf drinkers.In a paper accepted for publicationin

Sleep Medicine,theresearchers suggest that caffeine blocks production of the enzyme that drivesmelatonin production.

Because it can takemany hours to eliminate caffeine from the body,Ohayon recommends that coffeelovers switch to decaf after lunch.The authormentions" pick-me-up" to indicate that______.

A.decaf is acaffeinated coffee

B. coffee is astimulant

C. neurohormone canwake us up

D.melatonin levelsneed to be raised