The average U.S. worker with paid time off left 3.2 vacation days unused in 2013.

That data comes from research firm Oxford Economics, and was published in an article on travel industry news site Skift.

That information probably doesn't come as a surprise; various estimates say anywhere from 40 percent to two-thirds of workers fail to use all their vacation time. And a report last year from the Center for Economic and Policy Research showed that 25 percent of Americans take no vacation time at all.

But it rekindles questions about the extent to which taking vacation time matters, and if so, how you can encourage your employees to do so.

What's the Value of Time Off?

Time and again, studies show that time off has a positive effect on employee happiness and productivity. Meanwhile, many stories of invention and discovery have been born of time ostensibly spent on vacation.

Having said that, there's reason to doubt that ensuring your employees use all their vacation time really makes all that much of a difference.

For instance, mandatory vacation time--as required in some countries in Europe--doesn't directly correlate with employee happiness, according to The Atlantic.

Also, companies that institute unlimited vacation policies--a very attractive benefit for job candidates--grapple with an ironic issue: Their employees fail to take very much time off at all. I've previously encountered two companies who have said so--New Hampshire-based Dyn and Boston-based HubSpot--and the problem, so to speak, commonly comes up in articles about these policies. If you walk that back, you'd realize that the sorts of companies that are open to allowing unlimited time off tend to be the sorts of companies that employees like working for.

And with a number like 3.2 vacated vacation days--a number that registers as relatively small--it might be fair to consider whether it's all that big of a deal in the first place. For a number of people who don't use all their time off, maybe they're perfectly satisfied with their work-life balance as it stands. Couldn't it just be that those employees are happy enough to be at work?

Is It You or Is It Them?

There's a difference, however, between employees who are fine with giving up a few vacation days at the end of the year and those who would prefer to use them but don't.

That could happen for a couple of reasons, like if they're forfeiting those days because they're afraid that doing so will make them look lazy or hurt their prospects for a promotion. Or maybe they just work themselves too hard and don't know how to stop.

For their sake, you could consider stressing how much you value well-rested employees. Executive coach and author Tasha Eurich suggests encouraging employees who don't often take vacation time to split their time off into pieces; they may be more willing to take a three-day weekend here and there, rather than a full week off, for instance.

And if you're still having trouble getting employees out the door, you might also think about switching to a use-it-or-lose-it vacation policy if you currently allow for rollover days. While in name it seems heavy-handed, HR managers across the country say those policies better ensure employees use those days.

It's up to you to make sure your employees know they're entitled to their days off, and it can't hurt to encourage them to take them. But if you do so and they choose to leave them hanging, it could just mean they like coming to work. Hey, there are much worse problems to have.