This article originally published on August 9, 2015.

Weird fact: Mariah Carey sleeps about 15 hours a night. That’s about double the recommended minimum. The Grammy Award-winning pop siren snoozes more than half the day away for one reason and one reason only: so she can hit her ear-piercingly high high notes. Apparently it works, mostly. “I’ve got to sleep 15 hours to sing the way I want to,” she once told Interview magazine. Call it crazy. We call it career insurance.

Carey is far from the only famous personality to keep strange sleep habits. Donald Trump sleeps to the opposite extreme -- hardly at all. The bedhead-defying billionaire real estate mogul eeks by on just three to four hours of zzzs a night. “The Donald” says it lends him a competitive edge.

Related: The Surest Path to Success? Get Some Sleep.

Then there are the luminaries who fall somewhere in the middle when it comes to their sleep habits and nighttime routines -- people who tuck in a little more like us. Some celebrities sleep a full eight. Others scrape by on what little they can, depending on the day’s workload and what’s on the docket for tomorrow. One thing the rich and famous generally have in common is that we love to voyeuristically follow the days and nights of their lives, right down to how and when they hit the pillow (and, you betcha, who with).

Here’s a revealing glimpse into the bedtime routines of four of today’s most successful people. Go ahead and pry. You know you want to.

Related: The Surprising Sleep Habits of the Rich and Famous (Infographic)

1. Elon Musk

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The real-life inspiration for Tony Stark in Jon Favreau’s Iron Man movies purposely doesn’t indulge in too much caffeine before lights out, something he admits he used to do to his own detriment. The billionaire says he once guzzled up to eight cans of Diet Coke every day, on top of his customary two cups of coffee. He stopped when he realized he was getting “really wired.” Now he drinks “probably one or two [Diet Cokes], so it’s not too crazy.” Steering clear of the buzzy stuff for six hours before tucking in is best.

Musk, the visionary founder of both Tesla Motors and SpaceX, is a lifelong night owl, typically turning in around 1 a.m., then usually rising again at 7 a.m. During a recent Reddit AMA, the 44-year-old original PayPal mafia member revealed that he tracked how long he sleeps -- six to six-and-a-half hours, on average -- with his phone, a device never far from his workaholic grasp. So much for not taking tech to bed with you to bed, rocket man. Arianna Huffington, next up on this list, wouldn’t approve.

Related: 5 Things You Don't Know About Elon Musk

2. Arianna Huffington

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Arianna Huffington collapsed from exhaustion in 2007, two years after launching the Huffington Post. One night, after staying up late to catch up on work, she fell to the floor, smacked her skull on the way down and came to in a pool of blood. Involuntarily keeling over was a “life-changing” wake-up call, she says. She had no choice but to quit working 18-hour days and skimping on sleep.

The Greek-American media mogul is now an outspoken advocate for “ sleeping your way to the top.” And by that she means logging eight restorative, uninterrupted hours of shut-eye per night, which she strives to do every day. The recovering workaholic also bans all electronics from her bedroom, eliminating the temptation to zone out on social media and to check work emails (and stay awake stressing out over them). "I only have real books by my bed,” she told The Atlantic. “Not even Kindle editions. No iPad. Nothing. It's incontrovertible.”

Want to snooze like Huffington? Then try what she endorses as “ ultimate bedtime routine for better sleep.”

Related: Arianna Huffington Wants to Redefine Success. But Are We Ready to Listen?

3. Bill Gates

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Bill Gates is also an avid bedtime reader. The billionaire Microsoft co-founder makes a point of reading nightly, pouring over everything from inspiring biographies (Warren Buffett, Franklin D. Roosevelt) and exhaustive history tomes (about the development of vaccines and the atomic bomb), to deep philosophical dives and intellectual periodicals (The Economist, Scientific American). “I read an hour almost every night,” he told The Seattle Times. “It’s part of falling asleep.” But sometimes reading keeps him awake, too. “Like anyone who loves books, if you get into a good book, it’s hard to go to sleep.”

The 59-year-old college drop-out finds seven hours to be the best amount of sleep for optimal creativity the following day, also reports the Times. “Even though it’s fun to stay up all night, maybe taking a red-eye flight, if I have to be creative I need seven hours. I can give a speech without much sleep, I can do parts of my job that way, but in thinking creatively, I’m not much good without seven hours.”

For a full list of books Gates recommends, perhaps for your bedtime reading list, head over to his blog’s Books page.

Related: 10 Surprising Facts About Bill Gates

4. Barack Obama

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As you might imagine, our commander-in-chief doesn’t exactly practice stellar sleep hygiene. Obviously he’s kind of busy. To wind down from yet another stressful day at the helm of only an entire nation, Barack Obama doesn’t zone out on his BlackBerry in bed (yes, he still uses one of those). Instead, he dutifully stays up and takes conference calls with his senior staffers, often as late as 11 p.m., according to Politico. Noted for reviving the “night-owl presidency,” our 44th president habitually burns the midnight oil, reading briefing papers, doing paperwork and writing clear past 2 a.m. at times.

When the 54-year-old world leader manages to steal a few fleeting moments to unwind past dark, he reportedly reads good, old paper books, and not always about politics and domestic and foreign affairs. Long a fan of Jon Stewart, he says he occasionally tries to catch the The Daily Show (no pressure, Trevor Noah). But not before tucking in the FLOTUS for the night, that is after he tucks his two daughters in. “We do a lot of tucking,” Michelle Obama said on The View in 2012. “The girls will shout from bed ‘I wanna be tucked!.’ Then Barack comes in, he'll turn the lights out and we’ll talk. He’s like, ‘Ready to be tucked?’”

Related: The 7 Sleep Habits of Successful Entrepreneurs

As for how long the president sleeps, it varies night to night, depending on what’s happening in the world. However, we do know that his “ sleeping in” time is 8 a.m. When asked by TV and radio personality Ryan Seacrest if he ever hits “snooze button,” Obama fessed up that the White House operator calls and wakes him up. “"If I don't wake up the first time, they just keep on calling."

For more surprising facts about the sleep habits of the some of the best-known thinkers of our time and times past, tuck into this fun infographic, please, just not in bed on your smartphone, okay?

Related: Obama Wants to Make More Workers Eligible for Overtime Pay