America still has a vacation problem. Or a not-taking-vacation problem, to be more precise.

The second annual installment of Project: Time Off’s State of the American Vacation study shows that American workers left behind 662 million unused vacation days last year, 4 million more days than in 2015. More than half of American workers (54 percent) didn’t use all of their vacation days, slightly less than the previous year (55 percent).

The study included Project: Time Off’s survey of 7,331 U.S. employees who earn time off, as well as economic analysis conducted by Oxford Economics.

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Forfeited vacation days—time that cannot be rolled over, banked, or paid out—are down 8 percent, to 206 million forfeited days. Those forfeited days have a cost: Because the value of vacation days are tied to salary, that equals $66.4 billion in lost benefits, which equates to an average of $604 per employee. The study also found that the average American used more vacation time than last year, to a total of 16.8 days person, an increase of an entire half-day from the previous year. That increase delivered a $37-billion total impact to the U.S. economy.

The jump is huge, says Katie Denis, Project: Time Off’s senior director, who is the author of the annual report. Last year’s State of the American Vacation study introduced a new term, the “work martyr,” to describe employees who say that taking time off might indicate to their bosses and coworkers that they’re not making their job their number one priority.

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Are millennials really work martyrs?

Both millennials and their colleagues from other generations disagreed with that assessment, Denis said. “The pushback we got on that were people who were typically older demographics and who work with millennials and who said ‘that’s not my experience,’” Denis told Travel + Leisure.

Many factors are at play, she said, including seniority, position, budget, as are the number of vacation dates allotted to individual workers. “Millennial student loan debt is one of the factors, but so is home and car ownership.”

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This year’s study also found that men were more likely than women to use all their vacation time in 2016, up three percentage points from 2015 to 48 percent. While women said that vacation is “extremely” important to them, more so than men (58 percent to 49 percent), only 44 percent of women use all their time off.

“This disconnect between belief and behavior only worsens among Millennial women who, despite being more fervent believers in the benefits of time off than their male counterparts, take less time off (44 percent millennial women vs. 51 percent Millennial men) due to an overwhelming amount of reported guilt, fear, and work martyr habits,” the report states.

So who is to blame?

America’s corporate culture, for starters.

Bad bosses who send mixed messages are the most influential players in America’s vacation problems. Telling your employees to use all of their vacation dates, but then also saying “but I’m not going to use mine,” sends the wrong message.

“I think it’s perfectly easy to say take your time off, but I’m not going to,” Denis said. “It really does make a huge difference in employee behavior.”

Denis cited Mastercard as one example of a company that sent messaging to the workforce to encourage them to take time off, Denis said. The company’s president sent a video to the staff as part of its One More Day campaign. Within six months, company leadership found that it had reduced the number of unused days, from 4.2 per employee down to 3.9 days of unused vacation.

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How can you use all of your paid time off?

Start with planning. Anyone hoping to utilize all of their paid time off can take part in Project: Time Off’s Plan Your Vacation day, which is scheduled every year for the last Tuesday in January.

“The earlier the plan, the better deals you will get on flights and hotels,” Denis said. “But that’s fairly obvious. It’s Travel Planning 101.”