Calm was founded in 2012 by British entrepreneurs Michael Acton Smith and Alex Tew, with a stated mission “to make the world happier and healthier.” The company’s initial focus was on meditation, mindfulness, and mental wellness, with apps geared toward combating issues of stress, anxiety, insomnia, and depression. In 2017, Calm was plucked from a pile of two million apps and named Apple’s iPhone App of the Year. In a news release, Smith said, “You’ve heard of slow food and slow cinema. Now meet slow literature—half literature, half sleep, and totally relaxing.”

Headquartered in San Francisco, Calm is sold on a subscription basis. According to a company spokesperson, Calm has more than 1.5 million subscribers who each pay $69 for a year of access to all of the company’s digital in-app content. The sleep stories have been listened to more than 150 million times, and the app has been downloaded more than 54 million times.

The company’s addition of an app devoted entirely to sleep stories came from the realization “of how important sleep is to overall mental health,” Head said. The team at Calm worked with sleep psychologists to design sleep stories that work well because, he added, they “pull your attention in to focus on the story, so your racing thoughts are stilled. The stories help you wind down and ease off.”

With roughly one in four Americans suffering from acute insomnia each year, according to a recent study from the University of Pennsylvania School of Medicine, it’s no surprise that the sleep division of Calm has grown to include 120 stories, and has attracted celebrity narrators such as actors LeVar Burton, Stephen Fry, and Matthew McConaughey.

In January, Calm appointed Phoebe Smith its (and the world’s) first sleep storyteller in residence, certainly a desirable but challenging job, given that “it goes against everything a writer or narrator was trained to do,” Head said. Writers must ditch the idea of an arc built around tension, climax, and denouement, and narrators’ inflections must induce less excitement and drama, he explained. And they must embrace the idea that a successful sleep story is one no one finishes.

Smith is the author of more than 10 books, including The Wilderness Cookbook and Travel Writer’s Field Guide; a travel writer, broadcaster, and presenter; host of the Wander Woman podcast; and an editor-at-large for Wanderlust Travel magazine. “It’s a totally different kind of writing to anything I’ve ever done, and that’s why I like it,” she said about her work for Calm. “Anything exciting needs to go right at the start, and then it’s all about winding people down.”

Blue Gold, Smith and Calm’s most popular sleep story, has transported more than 15 million listeners across the world on a calming audio journey through the lavender fields and sleepy villages of Provence in Southern France.

Travel stories—with the rhythms of trains, the lull of cars in motion—are popular, but Calm’s team of seven writers offer stories for every age and taste, including reinterpreted fairy tales, nature stories, and general fiction. Each writer specializes in a particular genre, and with the exception of the adaptations of classic children’s tales—such as “Beauty and the Beast,” “Cinderella,” “The Little Mermaid,” and The Velveteen Rabbit—the stories are original.

“Variety is important,” Head said, noting that surveys have shown little consensus on favorites. Some listeners get comfortable with one or two that they go to every night. Head compares this to the experience of reading a book that you’ve read before: “It’s a bit more relaxing. You’re not on the edge of your seat, not that any of our stories keep you on the edge of your seat.”

The Center for Humane Technology ranked the Calm app the “world’s happiest app”—the single app that left users feeling happiest from spending time on it, based on data collected from a pool of 200,000 iPhone users. This is something that Head and his team at Calm take seriously. Recently, a father wrote to the narrator of a sleep story and told him that the book helped his son overcome terrible anxiety by listening to it every single night for more than a year. The boy, who won’t go to sleep with out it, is “doing really well,” Head said. “It is so meaningful and gratifying to hear how these stories help people with their struggles.”