“I probably started reading ultra hardcore about seven or eight years ago,” says Tom Bilyeu, an entrepreneur based in Los Angeles. “Ultra hardcore” means that Bilyeu reads everywhere: While he brushes his teeth, while he gets dressed, in the 30 seconds it takes to cross rooms in his house, he’s reading.

“My big secret is,” says Bilyeu, “I read in all those little transitional moments.” Plus, for the last eight years, he’s optimized his intellectual consumption by listening to audiobooks at three times the normal speed.

Audiobooks are the latest trend in book publishing. They’re part of the podcast boom, and they’re helping US publishers keep losses down as ebook sales from big-name companies continue to slump. What’s been around since the 1980s has a sleek new face, and today who’s listening, where, and why, offers a glimpse into a new reading trend sweeping the US.

Audiobook listening is growing rapidly specifically with 25- to 34-year-olds, thanks to a pernicious “sleep when you’re dead” mindset reflective of the young, aspirational, educated American: We are fearful of mono-tasking, find downtime distasteful, and feel anxious around idleness. Even when picking socks from a drawer, young workers feel better if information’s somehow flowing into their brains. And this is exactly the restless market that book publishers need.

A fast growing format

Audiobooks are booming audibly in the mobile age. In the US, growth of audio is stronger than any other format, according to the Association of American Publishers, which tracks revenue from 1,200 book publishers. And while audiobook unit sales numbers are still small (from January to September 2016, US traditional publishers sold $240 million in audiobooks, compared to $1.8 billion in hardcover books), the format’s growth has meant more and more publishers are putting their money in people’s ears.

“I am very bullish on audio,” Kristen McClean, executive director of business development for market trends company NPD Book. “This is on the top of my list in terms of things I’m watching.”

“What we’re seeing is something that goes beyond the simple ease of downloading,” she says. “I think there is a shift in consumption going on.”

A class of readers

Audiobooks are a way for people who were once big readers to keep up with their youthful curiosity. As they find themselves with less leisure time than they had in college, the gym and the car become opportunities to be stimulated. “I used to read a lot, and probably stopped when I went to law school,” says Jamie Brooks, a lawyer based in New York City. Now she listens to an audiobook a week, on average three hours a day, on the train to work and before bed.

Audiobook listeners tend to be slightly above average in terms of income and education compared to the rest of the US population, according to 2006 data (pdf), the most recent available from the Audio Publishers Association (APA). ”We find that our users are well educated, well paid, and successful,” says Beth Anderson, the executive vice president and publisher of Amazon’s Audible, the world’s largest retailer and publisher of digital audiobooks. “A huge number have masters and PhDs. They’re book lovers.”

“[Audio] gives us the opportunity to take time that would be dead time and make it into time that’s useful.”

But they’re also book lovers in a hurry. Mustafa Anil Kocak, a graduate student in electrical engineering at New York University, says he listens to 40 minutes to an hour of audio per day, during his commute, and to make his exercise regimen more efficient. “At the gym I feel like I’m wasting time,” he says. “[Audio] helps me not to think that way.”

One of the biggest use cases for audio listeners is the commute. But a sizable third say they listen while exercising, or while gardening or cooking. “[Audio] gives us the opportunity to take time that would be dead time and make it into time that’s useful,” says Michele Cobb, executive director of the APA.

Restless minds

Audiobooks mean we never have to be idle. They’re a cure to widespread restless mind syndrome, with its daily self-imposed nagging to make progress: Be more effective, says your productivity tracker. Do and learn more, says your to-do list. Optimize your to-do list, says your faddish new notebook.

Mobile technology helps. David Gross, a doctor and longtime audiobook listener based in Washington DC, recalls the trying process of procuring them 20 years ago: “There’d be a paper catalog, you’d call a phone number, they’d mail you the CDs, you’d keep it for a month, you’d mail it back,” he says. Today, downloads take two minutes, and apps make accelerated listening easy.

“It’s part of that obsessive, ‘Where else can I squeeze out another 10% of efficiency throughout my day and drink Soylent’ mindset.”

Podcast app Overcast offers “smart speed,” which shortens silences and pregnant pauses, so you don’t waste any time on the in-between. And Audible’s Whispersync for Voice lets you switch back and forth between listening and reading the ebook, so you can go from the car to your couch, or from the kitchen to your bed, without losing your spot. ”Audible makes it possible for you to read when your eyes are busy,” Jeff Bezos wrote of the feature in his 2013 annual report.

“It’s part of that obsessive ‘Where else can I squeeze out another 10% of efficiency throughout my day and drink Soylent’ mindset,” says Khe Hy, entrepreneur-in-residence here at Quartz. He recently confessed that during a period of his life where he was fixated on optimizing his time, he listened to audiobooks on double speed:

I spoke at a frenetic pace, bragging about all my time management hacks. I told her how about how I listened to audiobooks at 2.5 times their natural speed; how I’d created a BlackBerry shorthand language that included the most common English words; how I exercised by plowing through tens of thousands of burpees, praising their efficiency.

“That behavior was motivated by a hardcore scarcity mindset that there’s not enough time to do all you want to do in life,” he says now.

Bilyeu, whose mantra is ”Always Be Reading,” after the business truism, ”always be closing,” describes the moment he discovered he could listen faster than the normal pace: “The clouds parted; music started playing; angels were singing. Oh my God, you can speed this up!”

He started with 1.5x, and it was just at the edge of what he could understand. He stuck with it, and a week later he thought he must have accidentally turned it back to 1x speed, because the book didn’t sound fast to him. “I had just normalized it. So I bumped it up to 2x.”

Now, at 3x, it’s only the confused look of people who get in his car and hear the book begin to autoplay that reminds him of how insanely fast he’s “reading.” Bilyeu, who also speaks quickly, listens to 50 books a year. He doesn’t believe that he sacrifices any substance or style (he listens primarily to nonfiction), and says the speed helps him focus on the material, instead of giving him the space to daydream or wander.

A productivity trap

As publishers see more and more interest, they’re making more and more irresistible audiobooks, and paying big-name celebrities to help draw readers. Eddie Redmayne’s narration of JK Rowling’s fictional textbook, published by Pottermore, came out March 14, and is an Amazon bestseller. On February 14, Random House Audio released the aural version of George Saunders’s debut novel, Lincoln in the Bardo, with a 166-person cast, including Susan Sarandon, Don Cheadle, David Sedaris, and Lena Dunham. The movie version has since been sold to actors Nick Offerman and Megan Mullally, who both play characters in the audiobook.

At a February event in New York City, Saunders admitted it was the first time he was reading the book in public, and not a group of actors. He told Quartz the actors’ voices are the ones he hears now in his own mind.

In the way that film and TV adaptations imagine a world for the reader—the setting, characters’ faces, unspoken actions and glances—audiobooks do that through voice. In audiobooks, readers depend on producers and actors to predigest and interpret content for them. But the ease of that passive consumption can lead to another kind of dependence.

“Having our devices, which give us constant access to podcasts, audiobooks, radio livestream, we actually develop addictions to use them in a mindless way,” says Elana Feldman, assistant professor of management at UMass Lowell.

“Given that we have access,” she says, “People feel that they can and should use every moment of their time in a productive way. For a lot of people it means, ‘What could I also be doing right at this time?'”

Ticking boxes on a consumption to-do list in this way can actually impair longterm productivity and creativity in unforeseen ways, she warns. The mind can benefit when you listen to nothing at all. ”Creativity happens in non-conscious ways,” she says. “[Psychological] incubation can happen when you’re cooking, exercising, driving, when you’re not purposefully thinking about something.” It’s difficult to achieve that when you’re filling downtime time with mental stimulation, even if it’s a novel.

And in terms of productivity, listening to books in the gym may not be the best way to learn. Even though Feldman has never specifically studied audiobooks, she says, generally, “The concept of multitasking is really kind of a myth.” The cost of switching between simultaneous tasks exceeds the benefits.

But ultimately, of course, there are far worse addictions. Says Audible’s Anderson, “Listening to too many books or magazines is probably one of the less offensive vices one might have.”