Americans are leaving $52.4 billion on the table by not taking all of their paid vacation, new research finds.

“Americans are work martyrs,” according to the report by economic researcher Oxford Economics, “All Work, No Pay: The Impact of Forfeited Time Off” (pdf) — commissioned by the U.S. Travel Association and sponsored by American Express. “Tied to the office, they leave more and more paid time off unused.”

U.S. workers are forfeiting 4.9 out of 21 days of paid vacation annually, totaling 169 million days across the workforce, it concluded. The report is based on the “Monthly Current Population Survey” results reported by the U.S. Bureau of Labor Statistics (BLS) and a survey of 1,303 American workers.

This is the least amount of vacation time taken by U.S. workers in 40 years. From 1976 to 2000, workers used 20.3 days of vacation each year. Since then, the number has dropped precipitously, with workers reporting just 16 days used in 2013. And workers who forfeit 11 days-plus annually or more are more likely to suffer from stress, it found. “The drop-off started much earlier than the 2008 recession,” says Katie Denis, senior program director at the U.S. Travel Association. “Technology plays a huge part as people can work much more easily. Unplugging becomes more of a mental hurdle than anything else.” (Denis checks her email on vacation, but does not want or instruct those she manages to do the same.)

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Americans give up billions in lost vacations

Previous studies have actually been more pessimistic about the amount of time Americans take. U.S. workers only use 51% of their eligible paid vacation time and paid time off, according to a survey earlier this year of 2,300 workers who receive paid vacation. The survey was carried out by research firm Harris Interactive for the careers website Glassdoor. Adding to their stress: 61% of Americans work on vacation, despite complaints from family members; 1-in-4 report being contacted by a colleague about a work-related matter while taking time off, while 1-in-5 have been contacted by their boss.

In this regard, the U.S. is somewhat unusual in the developed world: Private American corporations are not obliged to give workers the day off on national holidays. Workers in the European Union are legally guaranteed at least 20 paid vacation days a year — and 25 or even 30 days a year in some countries. (Estimates vary on the actual number of days people are entitled to. Most American workers receive around 10 paid vacation days a year and six federal holidays, according to the Center for Economic and Policy Research, a nonprofit left-of-center think tank in Washington, D.C., even though the Oxford Economics study said many get 21 days of paid vacation.)

Workers in the U.S. may have more reason not to rock the boat at work than those in Europe. Most U.S. companies have an “employment-at-will” doctrine, meaning they can fire workers for no reason, unless they have a written contract, are a member of a labor union that has rules relating to conditions of employment or their firing is discriminatory.

But Denis believes the reasons are more emotional than legal, and reflect an insatiable drive to be critical to the company’s operations. “One person we spoke to in the focus group said, ‘We are all work martyrs and no one can do the job as well as I do,’” Denis says. “We sat behind the two-way mirror and we all thought, ‘That’s it.’”