Every summer, Americans selectively schedule their vacation time, typically taking a week or two to get out and enjoy the sun or a dreamy, far-off destination. But as the midyear heat wanes and the holiday season approaches, more of us should assess the vacation days we're not using—all 658 million of them.

That's how many paid days off were forfeited last year in the United States, according to Project Time Off, a research movement dedicated to improving work-life balance in the States. Project Time Off collects data from Americans to shed light on our vacation time problem every year. They found that 55 percent of Americans didn't use all their paid time off last year.

Yes, more than half of us are essentially giving away our valuable time and hard-earned money by forfeiting vacation days. This is especially troubling considering the fact that Americans already get less paid time off than many other industrialized countries, including Australia, Brazil, India, Japan, the United Arab Emirates, and most of Europe.

According to Expedia's 2015 Vacation Deprivation Study, the median worldwide number of paid vacation days (after holidays) is 25. The average American gets only 15. Americans forfeit about four of their days each year—which means we're only taking 11 days on average. That's almost one third of our already minimal earned time off—gone.

Here's a visual breakdown of Expedia's findings in each country. Americans forfeit the same amount of days that Malaysians do, and more than most of the other nations Expedia surveyed.

Those extra days we're forfeiting could have gone toward long weekends, down time with family, or even a midweek trip. So, why are we giving it up?

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It often comes down to guilt. Both Expedia and Project Time Off found that people forfeiting days say they think it looks bad to use them all—and that they assume bosses prefer employees to leave some days on the table.

"For some workers, vacation is a right, and for others, it's a guilty pleasure," Expedia's findings state. "Some workers also fear that their bosses will disapprove."

It looks like the guilt issue won't be improving in America any time soon. Project Time Off announced last week that they've found millennials overall experience more anxiety about taking days off, while older generations are more likely to see vacation as an earned right in the workplace. As more millennials enter the workforce and older employees retire, that mindset is likely to become more prevalent.

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"Compared to [Baby] Boomers, Millennials are at least twice as likely to find taking time off difficult," Project Time Off told me via email. "They don't want to lose consideration for a raise or promotion, don't want others to think they are replaceable, and want to show complete dedication."

Despite this, "69 percent of Americans report that their bosses approve of them taking their vacation time," Expedia reports. "Well above the global average of 56 percent."

Let's pledge to use our allotted days off from now on—or to at least be open with our bosses about them. After all, it is paid vacation.

How much time off are you giving up this year? Comment below.

More from SmarterTravel:

Read the original story: The Single Biggest Travel Mistake Americans Make Every Year by Shannon McMahon, who is a regular contributor to SmarterTravel.

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