Sept. 16, 2010 -- It has never been hard to find a good party in Sin City.
But the hottest time in Las Vegas these days happens to be at 2 o'clock on a Sunday afternoon.
Every Sunday for the past seven years, thousands of people have flocked to the pool at the Hard Rock Hotel and Casino in Vegas for a daylong party called REHAB. And new parties are now cropping up all across town.
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"We pop more bottles at the pools than at the club,'' one REHAB party guest said.
"[At] the nightclubs you can barely see anybody,'' another added. "Here you can see everybody ..."
Everybody and everything. On a typical Sunday, REHAB draws nearly 4,000 people, mostly young, attractive and wearing very little.
You know a party is popular when it gets its own reality show, "REHAB" on TruTV.
"Absolutely beautiful tans, beautiful people ... men," one woman said, holding a glass of champagne. "It's all good."
Good for partygoers and good for the Hard Rock's bottom line. Even in this sluggish economy, REHAB's revenues are up 10 percent from last year, a whopping 135 percent from four years ago.
"If you don't have a pool, you're not going to be relevant,'' said Jason Hancock, marketing director for Wendoh Media.
Here in the middle of the desert, where 120-degree days are as commonplace as slot machines, the party has suddenly, and inexplicably, moved outdoors. During the hottest hours of the day.
"You used to have people who come out for the night life and during the day, there was nothing for them to do," Hancock said. "People don't want to come out and gamble, the younger generation, they want to come out and party, and this gave them a place to go."
Pools and More Pools
And if imitation is the sincerest form of flattery, REHAB must be feeling pretty good these days.
"The clubs are the clubs, the pools are where everything's happening now," said Marc Jay, who produces pool parties. "In the last two years, it's really changed because hotels have sunk millions and millions of dollars into these pools and when you look -- one that just opened, the Encore Beach Club, I think they spent 75 million on that pool -- it's just incredible."
Jay, a Londoner, first traveled to Vegas for the night life a decade ago. "I think I spent 10 minutes at the pool," he said.
Now he lives here and this summer he's producing five of Vegas' biggest pool parties every week.
"Creating a great pool party is almost like baking a cake," Jay said. "You need different layers and your most important layer is your sprinkling of pretty people.
"So we have a team of people who would go out and find the good-looking models, or we invite girls that we find out in clubs ..."
Add to that landscaping, daybeds, bottle service, cabanas, top DJ's ... and drinks.
Eddie Perales, mixologist at Caesars Palace, said the pool bar on a hot day serves as many as 800 mojitos and about 3,000 specialty drinks called the Black Dragon.
Caesars Palace has eight pools, five of them new this year. The resort just finished a $60 million expansion on its pools alone.
"Nobody invests $40, $50, $60, $70 million in a pool just to get small returns," Jay said.
"When you see all these people checking in, especially the demographics 21-35, we know that they're coming to our pools and they're coming here to the cabanas, spending money on the daybed," Jay said. "It's all about spending money ... driving revenue to the hotel.
How much? Do the math.
"Just to come into these best pools, you're looking at $40-50 a person -- that's just to get in -- you don't have anywhere to sit or put your towels," Jay said. "So if you want a daybed or cabana, you could be looking at anywhere from $10,000 and below."
Party Time in Vegas: Sunday Afternoons
Some of the cabanas may be nicer than your living room and can run from $2,000 a day to as much as $15,000 on a holiday weekend.
Admission varies: The day we were there, getting into REHAB cost $50 for men and $30 for women. Daybeds start at $1,500.
But money can't buy everything, at least not for everyone. The line to enter the pools is often an hour or longer.
Once you're in, you're ready to sun, swim and even gamble. Where else can you find swim-up black jack? And under the waterfall?
ABC News' Kimberly Brown contributed to this report.