Why not use those last few vacation days?

Given an average of 14 annual vacation days, Americans left four days on the table over the past year, according to Expedia.com's 13th annual vacation deprivation study . That's twice as many as the previous year.

"No one retires wishing they'd spent more time at their desk," said John Morrey, Expedia's vice president and general manager, in a statement. "There are countless reasons that vacation days go unused -- failure to plan, worry, forgetfulness, you name it. But rested employees are more productive employees, so taking regular vacations may well help the company more than failing to do so."

Workers around the world cite a variety of reasons for not using those days. Some 25% say they like to stockpile days for possible trips in the future. Other reasons include difficulty of scheduling vacations (22%), getting paid for unused days (18%), worry they can't afford a vacation (16%) and failure to plan (15%).

But 8% point the finger not at themselves and instead at the atmosphere of their workplaces, saying they resist using their allotted days off because taking vacation days may be viewed negatively.

These figures and more were determined by Harris Interactive, which conducted this survey online from August 20 to September 12 in North America, Europe, Brazil and the Asia-Pacific region. In all, 24 countries were included, and 8,535 working respondents over age 18 took part. People had to be employed full time, part time or be self-employed.

Americans aren't the most vacation-deprived citizens. That honor goes to both Japan and South Korea. South Koreans take seven out of 10 vacation days, while Japanese workers take seven out of 18 vacation days.

Worldwide, working people get an average of 20 vacation days.

The French top the scale in vacation days, getting 30 off each year and -- get this -- taking all of them. Even so, 90% agree with the phrase, "I feel vacation deprived" despite the fact that they and other Europeans get more vacation time than most others. Even though they have fewer vacation days than the French, some 59% percent of Americans feel vacation deprived.

Norwegians are much less likely to pout about their lack of vacation time: Seventeen percent say they feel vacation deprived. Fewer than half of those from the Netherlands (41%), Denmark (39%) and Mexico (38%) feel the same way.

Once people do vacation, how successful are they at letting go?

Nearly half of those surveyed say they leave work behind "as soon as I leave on vacation," while 20% do when they arrive at their vacation destination. Only 10% can "never" truly relax while away.

Yet 67% of Americans remain connected to their offices via e-mail or voice mail, they say. More than 90% of French, Indian, Thai and Mexican travelers stay connected.