Ah, to find happiness at work -- that ever elusive goal many of us strive for, but don't always achieve.

You hope if you land that promotion or get that dream job offer, you'll finally be happy at work. And you do feel happy, for a little while. Then a few months in, you're back to your usual not-really-happy self. What gives?

Important-sounding job title, free food, unlimited vacation. These aren't the things that make people truly happy at work. (Though there's no denying that a cushy salary and chef-prepared meals are nice to have.) As tacky as it sounds, happiness starts from within.

New research from LinkedIn finds that people who genuinely enjoy their jobs have one thing in common: They make time to learn on the job. This could entail taking online courses, attending webinars, reading, or taking on new projects at work that help them expand their skill set.

The LinkedIn research surveyed about 2,000 professionals. They found that employees who spend time learning at work are:

  • 47% less likely to be stressed

  • 39% more likely to feel productive and successful

  • 23% more ready to take on additional responsibilities

  • 21% more likely to feel confident and happy

The happiest things in life are free

What's more, learning doesn't have to cost a thing. Library cards are usually free. So are Yale and Princeton's most popular classes, which are offered for free through the online learning platform Coursera. Many companies also reimbursement to their employees for professional development.

Heavy learners are happiest

People who are "heavy learners" -- meaning they spend five or more hours a week engrossed in learning -- reported the highest levels of happiness, productivity, and purposefulness at work. But that was only 7 percent of respondents. Realistically, most of us don't have an hour a day to carve out for learning. But even those who reported less time dedicated to learning each week reported positive results.

If you can't learn, it's time to go

The LinkedIn study also asked people the reasons they might leave a job. A fifth said the inability to learn or grow on the job would push them to look for a new one. This ranked higher than not getting an adequate raise, not getting promoted, or feeling overworked.

"As soon as you feel you're no longer growing, it's time to look for a new job," suggests HR analyst Josh Bersin, who wrote the summary of the survey results.

But let's get real. Work isn't going to be a happy place for everyone, no matter how much you invest in learning. Research even suggests that being unemployed is better for your health than working in a toxic job.