Bill Gates and Warren Buffett have given each other a few tips over the course of their friendship, which has lasted for more than three decades and counting.

One of those lessons is something Gates wishes he'd learned a lot sooner, to clear up his busy schedule — and possibly even made him happier and more productive.

"It took far too long for me to realize that you don't have to fill every second of your schedule to be successful," Gates posted on Meta's Threads app on Thursday. "In hindsight, it's a lesson I could have learned a lot sooner had I taken more peeks at Warren Buffett's intentionally light calendar."

Gates "had every minute packed" on his schedule when he was CEO of Microsoft, a job he held for 25 years before stepping down in 2000. He's admitted he was a difficult boss — one who wasn't shy about sending 2:00 a.m. requests to employees.

"I thought that was the only way you could do things," Gates told journalist Charlie Rose in an interview alongside Buffett in 2017. Gates finally learned to cut his employees, and himself, some slack after catching a peek of the Berkshire Hathaway CEO's personal daybook.

"[I] remember Warren showing me his calendar ... he [still] has days that there's nothing on it," Gates said, adding that Buffett's sparser schedule taught him an important lesson. "You control your time ... It's not a proxy of your seriousness that you fill every minute in your schedule."

"I can buy anything I want, basically, but I can't buy time," Buffett noted, during the same interview.

Buffett's method — essentially, "work smarter, not harder," — is backed by science.

Workers' proficiency steeply declines when they work more than 50 hours per week, a 2014 Stanford University study found. People who work up to 70 hours a week get the same amount of work done as those who hunch over their laptops for 55 hours, the research revealed.

You also shouldn't work too few hours in a week: People tend to be happiest when they can keep busy — no one wants to be bored — without working so much that they become over-stressed, research shows.

The optimum amount of daily free time on a person's schedule is up to 9.5 hours, one 2021 study found. That number likely feels unrealistic for most working adults, but setting aside more discretionary time can result in lower stress levels and long-term health benefits, the researchers noted.

Getting there may not be easy. It took Gates years to find a healthy work-life balance, he said in a commencement speech at Northern Arizona University last year.

"When I was your age, I didn't believe in vacations. I didn't believe in weekends. I didn't believe the people I worked with should, either," Gates said.

"Don't wait as long as I did to learn this lesson," he added. "Take your time to nurture your relationships, to celebrate your successes, and to recover from your losses. Take a break when you need to. Take it easy on the people around you when they need it, too."

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