Even Oprah Winfrey has joined in.
Family pyjamas by Burt Bees Baby are on her "Favorite Things" list.
Coordinating sleepwear, retailers say, is the new ugly sweater: Kitschy, extravagant and somehow irresistible.
Target said sales have grown every year since 2013, and this season it plans to offer 22 patterns, some with options for dogs and dolls.
Walmart, meanwhile, is expecting another year of "hefty" sales growth for its one-piece, zip-up pyjamas for adults.
New this year: Patterns depicting skiing polar bears and Santa riding a unicorn.
"This is definitely a peak year" for matching sets, said Debbie Horton, senior sleepwear buyer for Walmart. "People aren't just wearing them to bed anymore.
They're actually putting them on to go out and have fun."
At the PajamaGram, the holiday boom began in early September, when shoppers began planning photos for this year's Christmas cards.
The online retailer expects to sell 500,000 pairs of coordinating sleepwear this year for moms, dads, children - and of course, the family dog and cat.
Business has been so brisk that matching holiday pyjamas now make up 80 per cent of the company's annual sales, up from 15 per cent in 2005.
"It's great for business," said Stacey Buonanno, the company's brand director.
"It used to be that people would buy just one pair of women's pyjamas. Now they're buying four or five pairs for the whole family."
Prices start at about $20 (NZ$29) to outfit the cats and dogs, $30 (NZ$44) for children and $60 (NZ$88) for adults.
For a family of four plus a pet, that's $200 (NZ$293) - although Buonanno says that hasn't deterred buyers.
"We are seeing opportunities with this as a year-round business," she said. "Demand is just constantly growing."
The company has increased its lineup of designs from three to 27, and this year has licensing deals for Looney Tunes and Minions characters.
Other patterns include candy-cane stripes, snowflakes and color-your-own Christmas motifs, though classic red-plaid flannel remains the company's best seller.
Matching pyjamas have become such a hit on social media, Buonanno said, the company has begun asking customers to send in their photos.
It has received 5000 submissions over the past few years, many of which have ended up on the company's website and its catalogues.
"It's all about the family photos," said Justin Sonfield, a spokesman for the Company Store, where sales of matching flannel pyjamas have grown by double digits for five years in a row.
"In the retail world, it's what we call a home run."
It has been decades, Sharon Sweeney says, that she's been buying matching holiday pyjamas for her five children and six grandchildren.
These days, even if the extended family is apart during the holidays, they have three-way video calls on Christmas Eve so the family can see each other in their matching pyjamas.
"This is something we've been doing our whole lives," said her daughter, Susan Pennell, who lives in Newfoundland, Canada.
"It's just that now it's a lot more visible because everyone can see our pictures on Instagram."
Each photo posted on Instagram - or Facebook or Snapchat - ends up reaching dozens, if not thousands, of people, which furthers the wildfire-like spread of matching pyjamas, said Jonah Berger, a marketing professor at the University of Pennsylvania's Wharton School of Business.
"It used to be that maybe your uncle's family did this on their Christmas card, but you'd only see it if you were on their mailing list," said Berger, author of the book Contagious: Why Things Catch On.
Now, he said, "It's a classic case of internet one-upmanship. Who can come up with the best matching pyjamas and show the world that they're a good parent?"
Joshua Pease, for one, isn't feeling that pressure.
He says he was caught off-guard when, a few weeks ago, his wife casually mentioned that maybe it'd be fun to dress their family of four in matching pyjamas.
Pease, a writer in Castle Rock, Colorado, was not amused. He's never liked uniforms, he said, and something about coordinating sleepwear just seems, well, corny.
"I love my family, and I'd do anything for them," he said. "But matching pyjamas? That's where I draw the line."
Jeff McGurren used to feel the same way.
But he caved last year after his wife, Jacqui, told him he'd ruin Christmas if he didn't join in.
So he sheepishly put on a pair of reindeer-emblazoned pyjamas and posed with their then-5-month-old son and two dogs for their annual holiday card. (One of the dogs also wore antlers).
"You can tell by the look on his face he wasn't too happy about the family photo," Jacqui said. "But he is a good sport [and] agreed to do another one this year."
Which means, she said, she'll have to go shopping.
The family recently doubled in size, with the addition of five chickens.
It would be a lot to corral for a holiday photo - even if the animals co-operate.
It was hard enough, she says, getting her dogs to co-operate last year.
"They were totally skeptical and didn't talk to me for at least an hour after the shoot," she said, adding that Sprinkles tried to eat her pyjamas off, while Tina looked a bit ashamed in her outfit. "It was a rough day for everyone."
But that doesn't mean she isn't trying again.
"I really want to do one this year with the five chickens matching," she said, adding that she's already found fowl-friendly Christmas sweaters. "I just love the cheesiness of it."
