Who's keeping American workers from using the time off they accumulate? Mostly, themselves.

That's what a study from the U.S. Travel Association tells us. The association, with GfK Public Affairs and Corporate Communications, asked more than 1,000 U.S. workers who toil at least 35 hours a week about their use of paid time off. Overall, 40 percent said they didn't use all their PTO, supporting earlier studies on the subject. Here's what else they found:

• 40 percent said they faced "a mountain of work" when they returned from time off and would rather not leave;
• 35 percent said "nobody else can do my work," so they didn't use all their time;
• 33 percent said they "couldn't afford to take" all their PTO (even though it's paid);
• 22 percent said they didn't want to appear to be replaceable by taking time off.

"Americans suffer from a work martyr complex. In part, it's because 'busyness' is something we wear as a badge of honor. But it's also because we're emerging from a tough economy and many feel less secure in their jobs," said Roger Dow, president and CEO of the U.S. Travel Association. "Unfortunately, workers do not seem to realize that forfeiting their vacation time comes at the expense of their overall health, well-being and relationships."

This complex exists despite survey results that showed that 95 percent of C-Suite respondents agreed it was important for the troops to take all their PTO. But employees aren't so sure that's the truth. When the researchers asked them about company attitudes toward time off, they got these responses:

• 67 percent said their company "says nothing, sends mixed messages about or discourages using their PTO;"
• 33 percent of bosses said they never (19 percent) or rarely (14 percent) talk about the benefits of time off with the troops.
• When they do take time off, nearly half of respondents said they respond to emails, and 29 percent return work-related phone calls. And bosses set really poor examples along these lines: 37 percent said they unplug completely from work while taking time off, compared to 74 percent of employees.

"While the survey revealed a number of barriers to taking time off, it may have also uncovered the silver bullet," said Chris Moessner, vice president of research at GfK. "'Use it or lose it' policies are tremendously effective at getting employees to use the time off they have earned."