NEW YORK (CNNMoney.com) - When it comes to taking vacation days, a third of U.S. workers apparently aren't members of the clean-plate club.

A survey conducted by Harris Interactive and commissioned by the travel site Expedia found that 33 percent of us leave vacation days on the table. The average number of days workers forfeit: four, up from three in last year's survey.

And it's not as if the United States leads the world in vacation days granted. The average is 14 days in 2006, according to the survey, which is far below that of many other industrialized countries such as Great Britain (24 days) or France (39 days).

Of course, travel businesses have an interest in reminding you that you let vacation days go begging, since there's a good chance they could profit from your disgruntlement should you decide to make full use of all the time to which you're entitled.

But the truth is a vacation needn't involve a trip anywhere, and Expedia's survey isn't the only one to conclude that many U.S. workers forego paid days off.

A study released last year by the Family and Work Institute (FWI) also found that about a third of workers (36 percent) did not plan to use all their vacation days. And 37 percent said they never taken more than a week off at a time.

There are a host of reasons working adults offer for why they may give up vacation time or take their time only in short spurts:

• They prefer the money if their employers agree to pay them for the days they forfeit.

• They experience increased stress at work both in the run-up to vacation and immediately upon their return.

• They're just too busy to leave.

• They're afraid their boss will hold it against them.

All seem reasonable given how demanding the workplace can be. But they're less so when you consider what you're really forfeiting.

In part the answer is "having a life" - you know, that bit about doing something other than your job until your last dying breath, and improving your health while you're at it.

And in part the answer is "give your career a boost."

Yes, it's important to relax and rejuvenate yourself for personal reasons. But in conducting interviews for his book, "How to Succeed in Business Without Working So Damn Hard," author Robert Kriegel found that the "best ideas people ever get are when they're away from work."

If you're like a lot of people, your main job is to respond to every little thing that comes up, much of which is beyond your control, he said.

There's little opportunity to step back and consider how things might be done better or to dream up innovative ideas. (And no, those two-day 'offsites' in windowless conference rooms don't count as serious step-back time.)

Fears that things will fall apart in your absence often are unfounded. In most instances, "you get back and nothing has changed," Kriegel said.

And if your boss is the kind who subtly discourages you from using "too much" vacation time, you might be able to get around that first by identifying what his or her potential concerns are and then covering all the bases that can allay them, Kriegel suggested.

Let your manager know how you can be reached if need be. But if you've adequately covered your bases, it's likely the office will take care of things in your absence.

To ward off the depression that can hit when you return from a vacation, think about what motivates you, Kriegel said. He suggests planning to have a few things that you love doing on your agenda when you return.

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