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STEVE INSKEEP, HOST:

It's MORNING EDITION from NPR News on a Monday morning. Good morning, I'm Steve Inskeep.

Today in Your Health, new treatments for Hepatitis C are curing many patients. First, let's get a dose of caffeine. Visit a college campus and you're likely to find that nearby coffee shops are empty in the morning - who's awake then - and packed at night. New survey data show that coffee drinking at the nation's colleges is way up over the past decade.

Now, I'm going to step out for a moment and get a cup of coffee. But while I'm gone, listen to NPR's Allison Aubrey, who reports on what that coffee drinking means for students.

ALLISON AUBREY, BYLINE: Researchers at the market research firm NPD Group track trends in what Americans of all ages eat and drink, including caffeinated beverages. Harry Balzer, who oversees the research, says one striking change he's documented is that college-aged kids have cut way back on caffeinated soda.

HARRY BALZER: But what really was the trend was they traded their caffeine from cola to their caffeine from coffee.

AUBREY: Balzer says consider the change over the last 10 years.

BALZER: In 2002, about 25 percent of 18-to-24-year-olds had coffee sometime in a two-week period.

AUBREY: But by 2012 the percentage of young adults drinking coffee hit 39 percent.

BALZER: Not quite a doubling, but it's up. It's an explosive growth in the consumption of coffee.

AUBREY: For evidence of this trend, I hit a coffee shop on the campus of George Washington University, where in mid-afternoon the place was packed.

ARTURO LICHAUCHO: This is nothing for 2:00 in the afternoon. The library is right there. It's next door, so...

AUBREY: Senior Arturo Lichaucho. He says people hang out here before hitting the books. Busiest time, he says, is early evening. When I asked him and his friends if they were surprised to hear of a significant jump in coffee consumption among their age group, they laughed.

LICHAUCHO: No.

(LAUGHTER)

KALEY INDECH: Why?

AUBREY: Student Kaley Indech says for lots of college students overstimulation is a way of life.

INDECH: Especially with like always being on our computers and stuff, and it causes, like, you to feel the need to re-energize with caffeine.

AUBREY: Her friend Richelle Gamlam, a senior, says the intense demands on their time is at play too. She's got an internship this semester.

RICHELLE GAMLAM: I'm working over 20 hours a week, five classes, involved on campus. So just the timing, like, a lot of times, you know, if you're getting less sleep, you really - I need that coffee to make it through class or get through work.

AUBREY: And why not drink more coffee? Recent studies have linked it to a host of good health effects, including a decreased risk of dementia and a lower risk of depression - at least among women.

But experts say there is one downside that's often overlooked. A 12-ounce cup of coffee from Starbucks can contain four to five times the amount of caffeine found in a Diet Coke of the same size. And this means coffee can get in the way of good night's sleep.

Richelle says it's happened to her.

GAMLAM: Yeah, you're really tired. And then you get in bed and you're, like, oh wait, I can't fall asleep now.

(LAUGHTER)

GAMLAM: But it doesn't happen very often.

AUBREY: Professor Amy Wolfson, of College of the Holy Cross, studies the effects of caffeine on sleep among adolescents.

AMY WOLFSON: Yes, absolutely. There are absolutely negative implications.

AUBREY: For instance, studies link high caffeine use to decreased REM sleep.

WOLFSON: We know that REM sleep is needed and has positive implications for memory consolidation and learning. And so if you think of the average college student having less sleep or poorer quality sleep, is likely to have negative implications for academic performance.

AUBREY: So what's the smartest way to consume coffee to get that boost without busting good sleep? Bruce Goldberger, a toxicologist at the University of Florida, says it's about timing. Consider this: The typical half life of caffeine is about four to five hours. This means it stays in the body about 10 hours.

BRUCE GOLDBERGER: If someone has a cup of coffee at 7:00 p.m., that caffeine that they've ingested, is still in the body affecting the central nervous system when they're going to bed.

AUBREY: There's a lot of variation from person to person. And due to genetic differences, some of us metabolize caffeine a lot more quickly than others. But as a rule of thumb, if you want to go to sleep by midnight and stay asleep, it's probably best not to drink coffee after 2:00 p.m.

Allison Aubrey, NPR News.

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