Vacation deprivation not healthy. In fact, it's costly.

Americans are vacation-deprived -- even though they earn fewer vacation days on average than workers in other developed nations, they still leave an average four days unused.

An Expedia.com survey released earlier this year found nearly 50 million Americans -- about a third of the workforce -- don't take all their vacation, some taking only a day or two.

The average U.S. worker earned 18 vacation days compared with 37 for French workers, who left just an average two days unused. Forty-six percent of U.S. workers say they work more than 40 hours a week, but only 38 percent say they use all their vacation days, the survey found.

"I think there are multiple reasons for Americans not taking more time off," Dr. Madhukar Trivedi, a professor of psychiatric research at the Southwestern Medical Center in Dallas, told United Press International in an interview.

"There is a perception that people who work long hours -- those that are the first ones at work and the last to leave -- get more credit than those who work fewer hours; there is more fear in the workplace after the recession; due to the recession layoffs there are fewer people to replace a vacationing employee so some might not be able to take time off or they dread the pile of work they have to face when they return.

"Of course, working the longest hours doesn't mean the person deserves more credit than those who don't -- but that is the perception. It is not the amount of time a person works, it is how effectively they work, but this is the way people see it."

Another reason might be cultural: Unlike other countries, the United States has no laws mandating vacation days.

"The bigger issue is this emphasis on working long and hard. (It) is a cultural influence -- not an overt decision -- and for some this approach and lifestyle might help account for the high performance America has in the economy," Trivedi added. "For some working all the time gives them tremendous benefits."

Respondents recognize vacations are good for professionals, leaving them feeling better about their jobs, more productive, rested, rejuvenated and reconnected with their families.

"Because people have been taking fewer vacations, they may forget how much better they feel when they return to work after a vacation," Trivedi said.

There are numerous studies on how vacations or time off improve employee health, social relationships and productivity. In fact, people don't have to leave town -- any time off has health benefits.

Karen Matthews of the University of Pittsburgh's Mind-Body Center surveyed 1,399 study participants participating in other studies on breast cancer, cardiovascular disease and other conditions.

The 2009 study found leisure time contributes to more positive emotions, fewer negative emotions, less depression, more life satisfaction and more meaning in life. It also left people feeling more religious or spiritual.

The study found those who took time off had a larger network of friends and family -- a support network other studies have found key to survival of diseases like cancer.

Matthew's study found tangible health benefits of lower blood pressure, lower stress hormone levels and smaller waists -- all of which contribute to less risk of diabetes and heart disease.

The landmark Framingham Heart Study, which began in 1948 to identify the risk factors that contribute to cardiovascular disease by tracking men and women over a long period of time, found the more frequently the men vacationed, the longer they lived. The study also found women who didn't take vacations were as much as eight times more likely to suffer from heart disease than women who took two vacation breaks a year.

Dr. Munro Cullum, a professor of psychiatry at the University of Texas Southwestern Medical Center in Dallas, said some level of stress can help people keep going and hasten projects and accomplishments, but if stress becomes too much, it can also result in negative physiological reactions that can lead to illness.

Exercise, a healthy diet and adequate sleep are important to maintain health, but people need some down time, to allow their brains to work "offline," Cullum said.

Cullum said even taking a short break can be rejuvenating, but significant de-stressing might take several days "just to get ourselves used to the idea of relaxing."

"Getting away for a vacation allows us time to simply play and leave the 'baggage' behind," Cullum said in a statement.

So why don't Americans take more vacations?

Tony Schwartz, author of a Harvard Business Review article "The Productivity Paradox: How Sony Pictures Gets More Out of People by Demanding Less," says fewer U.S. workers are working and those who are working are working fewer hours than before the recession.

"If colleagues are being laid off and cut back, we can't help worrying that you might be next and we push ourselves harder, we get more done, which sounds like good news and certainly explains higher productivity," Schwartz said. "But just as you'll eventually go broke if you keep withdrawing from your bank account without offsetting deposits, you will also ultimately burn yourself out if you spend too much energy too continuously at work without sufficient renewal."

Schwartz, whose company addresses work performance and the problem of employee disengagement, said a comprehensive study by Ernst & Young showed the longer the vacation employees took, the better they performed.