For a group of sixth-grade girls growing up in Chicago, it was the best kind of summer night.

One friend brought the Cheetos, another brought a Blockbuster DVD. Someone else brought a list of phone numbers for the boys in their class that they planned to prank call. Together, they descended into somebody’s parents’ unfinished basement, carpeting the floor with pillows and fleece blankets. “The space transformed into our own cocoon of friendship,” said Christine Schmidt, who writes a newsletter on friendship, thinking back on her childhood. Once the lights were off, everyone tucked into sleeping bags, they whispered their secrets: conflict at home, deeply-felt insecurities, the name of a crush. That kind of night, Schmidt told me, was “magic.”

Schmidt, now 23, hasn’t had a sleepover in years.

Sleepovers are an important fixture of childhood friendships, particularly for girls, said Deborah Tannen, author of “You’re the Only One I Can Tell,” a book on the language of female friendships. But by the time women reach their late teens or early 20s — usually by graduation for those who go to college — the sleepovers stop. As adults, friends typically meet up for happy hour, or a meal — tidy blocks of time, just long enough to catch up on major life events that have occurred since the last tidy block, weeks or months before.

“It ends up feeling like a work meeting,” said London-based writer Gwendolyn Smith. “Which is really kind of sad.”

When the era of the socially-acceptable sleepover ends, women, in particular, lose out on something important. Among female friends, Tannen says, “closeness” is often created and measured by the “exchange of information”: The more personal stories women share with each other, the closer they feel. And there is no better setting for information exchange than a sleepover. (Men, on the other hand, are more likely to measure closeness by the type and number of activities they do together, according to Tannen.)

“There is something about the quiet of night,” Tannen says, “... the extended amount of time, the knowledge that you’re not going to be called away, that your phone isn’t going to ring … that makes people more comfortable sharing.”

When Smith, at age 26, invited seven of her friends to join her for a slumber party, they were in their pajamas, eating potato wedges, by 7 p.m. After they put on their face masks — hair pulled back, cucumber circles balanced on their eyes — they’d planned to watch a movie. But everybody just wanted to talk.

“At first it was just the regular catch-up conversations: ‘What’s going on at your job? Are you enjoying who you’re seeing at the moment?’ But then the conversation moved beyond that,” said Smith. Several people talked openly about their struggles with mental health, Smith wrote in Grazia; one woman spoke about the hidden challenges of a job everyone had assumed was perfect.

The routine of a slumber party can be powerful, too. The particular acts that make up a sleepover — getting ready for bed, falling asleep, moving around the kitchen in the morning — send what Tannen calls a “meta message” of closeness to everyone involved. Because we typically move through these kinds of routines alone, with a romantic partner or with members of our immediate family, she says, we instinctively feel closer to friends who experience them with us.

Women do seem to recognize the importance of sleepovers in adulthood. The everyday slumber party has been replaced by the “girls’ weekend,” says Emily Langan, a professor who specializes in friendship at Wheaton College. The concept has become so popular that hotel chains are rushing to cash in on it, offering special packages with eye masks and copies of “Clueless” and “Dirty Dancing.” Once every couple of years, Langan, 46, and a group of her oldest friends — now spread out across the country — hold their own girls’ weekend at one of their homes. They find babysitters for their kids and leave partners behind. Last time, they stayed up into the early morning. Langan laughed until her belly hurt.

But girls’ weekends are not quite sleepovers, Langan says, because they’re not “normal life.” When a group of friends goes away together, it’s usually the culmination of months of planning. It feels “momentous,” says Smith. But spending a night with friends shouldn’t have to be that big of a deal.

The biggest impediment, of course, is time. Sleepovers are long. By the end of young adulthood, people are faced with a “role crunch,” says William Rawlins, a professor of friendship and communication at Ohio University. After the “golden age” of friendship in the early 20s — when friends lean heavily on each other as they figure out who they are and who they want to be — people assume new responsibilities that they need to prioritize: a romantic partner, a demanding job, kids, aging parents. “The next thing you know,” he says, “you don’t have the time for friends that you once had.”

“In my normal life, I don’t have time to get more than a cup of coffee with you, let alone have this long, uninterrupted period of time together,” Langan says, a little sadly.

As a woman hosting a sleepover in her late 20s, Smith told me, she felt “a bit naughty.” With everyone so busy, and proud to be so busy, she says, it seemed decadent — impolite, almost — to suggest a social activity that lasted all night and into the morning. But the result, as Smith describes it, was worth 10 happy hours.

And maybe that’s the answer: Replace a few weeks’ worth of brunches and happy hours with one big slumber party. Even in childhood, Schmidt says, sleepovers are a “rebellion” against routine: against bedtimes, healthy breakfasts and eight-hour sleep schedules.

“You’re bending the rules, and it feels like it could last forever.”