Maybe you noticed all those kids entering the museum while you were on your way out, or perhaps got firsthand experience schlepping and sleeping there with your own son's Boy Scout group.

Indeed, pitching makeshift campsites beside a mythical Sphinx, a limp-armed T. rex, or a pulsating heart has been popular for the last 10 years, especially since the Night at the Museum film franchise widened appeal. (The third will be released in December.)

But it's almost always been a thing just for kids - until now.

In response to adults clamoring for a turn to pack their own toothbrushes, don eye masks, and catch some Z's, museum directors in local landmarks looking for more exposure are holding adult sleepovers, a trend that seems to be gaining ground. Part curiosity, part thrill-seeking, it's a novel experience for the over-21 crowd who want an alternative adventure after dark.

For 20 years, Jennifer Hagopian of Maine has been a fan of the Mütter Museum, where a maze of displays house medical oddities: cancerous tumors in jars, slices of Einstein's brain, and a death cast of Chang and Eng Bunker, conjoined twins who fathered a total of 21 children.

So when she heard in November about the Center City museum's first-ever adult sleepover, she was more than eager to sleep with 20 strangers while locked in an institution.

Hagopian, 43, went with her boyfriend, Adam Cogswell, 35, and two friends, Carrie Vinette, 35, and David Donovan, 35. After a seven-hour drive, she joined a mostly millennial group - though some lodgers were in their 50s and 60s. Others had come from Michigan and Connecticut.

She opted for comfy sweats, but neighboring bedfellows zipped into footie pajamas. Hagopian, a member of the catering crew for the WWE, says the night gave her "cool bragging rights" when she returned home. Two months later, she went back and slept over again.

"The place is so creepy, but it was even more so late at night, especially in the library," she says, describing the stacks where rare volumes are kept. "You could smell the old books. I had such a feeling of exploration."

J Nathan Bazzel, spokesman for the College of Physicians of Philadelphia, home of the Mütter, describes the sellout sleepovers as intimate and like glamping, or glamorous camping. For years, touring patrons asked about the possibility. Others posted requests on the museum's Facebook page.

"There's this mystery of what might go on in the museum when everyone leaves," Bazzel says. On Friday, the museum will hold its third adults-only sleepover.

The night begins at 6 with gourmet pizza and a sci-fi movie - The Sixth Sense was shown at earlier events. There's a flashlight tour, a seance in the Gross Library with illusionist Francis Menotti, and ghost stories with s'mores around a campfire. Everyone brings sleeping bags or air mattresses and sleeps in the galleries. After a night of "unrest," as the website declares, guests receive a buffet breakfast on the veranda. Cost is $200.

Oh, and there's alcohol - a three-drink max of wine or beer.

The booze and having access to dark and concealed passageways apparently stimulate some mortals to act out a fantasy, say, beyond snoozing next to an exhibit of bone and spinal deformities.

Asked to elaborate, Bazzel pauses, then answers carefully. "Unlike kids, who stay where you tell them, adults tend to sneak off. We are prepared if it happens. We have a security team that will be on the premises."

In fact, rumors sounded that during the first adult sleepover for 150 in August at Manhattan's American Museum of Natural History - which sold out in three hours - a couple performed their own private feat in their seats during the live animal show in the auditorium.

Jill Sybesma, manager of adult and strategic programs at the Academy of Natural Sciences of Drexel University, is planning its first adult sleepover for 100 on March 7, costing $165 for members, $190 for nonmembers.

The academy certainly has the template down: Since 1986, it has had thousands of kids spend the night in various programs. The adult affair will kick off at 6:30 p.m. with a happy hour (four-drink max of beer or wine), followed by tours and stories about the dioramas. Case in point: The antler rack on the moose was swapped for larger ones from another moose in 1933. The taxidermist wanted a more striking display.

There will be a fajita dinner, live animal shows (a skunk, a hedgehog, or birds of prey), and a screening of an "awesomely awful" movie, followed by mocking commentary. Everyone leaves the next morning by 9, after mimosas and the oatmeal bar.

Sybesma is aware of some of the big-kid issues and will have measures ready, too. "We do a lot of proms and corporate events. You'd be surprised at what people try to do."

Whether you're new to art history or a renowned paleontologist, visiting museums can be overwhelming, says Mark Kennedy of Museum Hack in Manhattan. Museum Hack leads subversive and creative tours at famous museums for corporate and private groups, like the Metropolitan Museum of Butts tour, held for bachelorette parties, which showcases 6,000 years of derriere history. "We like to shake it up."

But undoubtedly, the real shaking takes place during ghost-hunting overnights at historic Fort Mifflin on the Delaware River, long considered a hotbed of paranormal happenings. "They're called sleepovers, but sleep is optional," quips executive director Elizabeth Beatty.

On Halloween, those paying $90 will be able to accompany ghost-hunting pros equipped with laser grids, electromagnetic field meters, and temperature gauges in search of apparitions like the long-haired blacksmith, the smiling dwarf, and the plucky Union soldier who has been said to give tours to trusting visitors.

Dewey Blanton of the American Alliance of Museums in Washington said adult sleepovers were increasing in popularity across the country as another option to entertain members and donors. And to raise money.

The Battleship New Jersey in Camden has been selling out its youth encampments, held Fridays and Saturdays, since 2002. Capacity is 400 people, and stays cost $62. There have been a few adult sleepovers, mainly veterans' reunions.

Jack Willard, senior vice president of marketing and sales, says the sleepovers are a major moneymaker.

"People get to pretend they're in the Navy," Willard says, "and we put money back into the ship. Everyone's happy."