And now, a word from our sponsors. A very brief word. TV commercials are shrinking along with attention spans and advertising budgets. The 15-second ad is increasingly common, gradually supplanting the 30-second spot just as it knocked off the full-minute pitch decades ago.

For viewers, it means more commercials in a more rapid-fire format. For advertisers, shorter commercials are a way to save some money, and research shows they hold on to more eyeballs than the longer format.

“It used to be that the most valuable thing on the planet was time, and now the most valuable thing on the planet is attention,” says John Greening, associate professor at Northwestern University’s journalism school and a former executive vice president at ad agency DDB Chicago.

So instead of seeing a lengthier plot line, viewers are treated to the sight of, say, the popular “Old Spice man” riding backward on a horse through various scenes for just 15 seconds.

Or the “most interesting man in the world,” the suave, rugged, Spanish-accented character pitching Dos Equis beer, appearing just long enough to turn his head and weigh in on the topic of rollerblading. (Verdict? A deadpan “No.”)

The number of 15-second television commercials has jumped more than 70 percent in five years to nearly 5.5 million last year, according to Nielsen. They made up 34 percent of all national ads on the air last year, up from 29 percent in 2005.

Commercial-skipping digital video recorders and distractions such as laptops and phones have shortened viewers’ attention spans, says Deborah Mitchell, executive director of the Center for Brand and Product Management at the University of Wisconsin. Viewers are also watching TV streamed on sites like Hulu, where advertisers have less of a presence.

So companies figure: “Why spend money on anything longer anyway? Plus, if they’re going to skip our ads, at least we have a better chance of them seeing something if it’s really short.”

Fifteen-second ads cost about the same per second as longer ones but, of course, cost half as much. A 15-second ad on network TV cost about $20,000 on average last year, according to Nielsen.

“It becomes a very seductive thing to get your message out there at half the cost,” says Mike Sheldon, CEO of advertising agency Deutsch LA, a unit of Interpublic Group.

On average, about 5 percent of an audience viewing a 15-second commercial will give up on it. The number jumps to about 6 percent for 30 seconds and 6.5 percent for 60 seconds, says Jeff Boehme, chief research officer for Kantar Media.

Previously, 15-second ads were mostly edited versions of 30-second spots, but that’s changing. Advertisers are making shorter commercials as stand-alones. The length is ideal to remind people of products, stores or prices, but not to introduce them.

More than half of commercials run by packaged-goods companies and 60 percent of fast-food ads are 15 seconds, according to Kantar. The advertisers simply show a picture of the products, flash a price and the brain knows what the marketer means.

Take the new campaign for Burger King, which is selling its breakfast options. A 15-second ad airing now features a mailman walking down the street carrying a plate of eggs, pancakes and hash browns. There’s no verbal description of the product. Instead he sings: “Did you know that breakfast was served at Burger King? The ultimate breakfast platter. That’s what I call delivering.”

The shorter ads also mean marketers can be on the air more frequently, even within the same commercial break. For example: During a recent episode of CBS’ “How I Met Your Mother,” viewers were bombarded with five brief ads in just a minute and a half, including two spots for Dunkin Donuts sandwiched around a more traditional 30-second ad for Aetna.

The repetition helps beat messages into viewers’ heads. That’s why Anheuser-Busch would rather air four 15-second ads for Select 55, its 55-calorie beer, than one 60-second ad, says Keith Levy, marketing vice president for the St. Louis subsidiary of Anheuser-Busch InBev.

“With Select 55, we were trying to establish the notion that this was the lightest beer in the world,” he says. Simple commercials featuring a bottle that floats on air don’t need long to drive home that message.

Big advertisers are driving the shift. Procter & Gamble, the maker of Crest toothpaste and Tide detergent and the world’s biggest advertiser, doubled its number of 15-second ads to more than 299,000 last year from the year before.

Walmart, the world’s largest retailer, has increased its use of 15-second ads nearly 30-fold to 148,000 last year from only about 5,700 in 2005. The retailer plans even more this holiday season.

Shorter ads can be just as effective as longer ones. Viewers can form new associations — say, knowing about a discount — in a few seconds and then recall that information in just one second, Mitchell says. People can’t help soaking up the message.

“When things are working that fast, you can’t tell yourself, ‘No, I’m not going to think about that,“’ she says. “Your brain lights up so you don’t have a choice.”