Americans are taking fewer vacations than in years past, with more than half of workers leaving some vacation time unused in 2015, a new study finds. The report from Project: Time Off estimates that the lost vacations deprived the U.S. economy of $223 billion in spending on restaurants, home-improvement projects, hotels and other travel.

According to the study, the problem stems from smartphones and other technology that keeps employees tethered to their jobs. “The constant connectivity makes us feel so integral and indispensable that we have a harder time getting away,” Kate Denis, senior director of Project: Time Off, told the Wall Street Journal.

Last year, workers used 16.2 vacation days on average. That’s close to four days fewer than employees used on average each year between 1976 and 2000. And 55% of those surveyed left days unused in 2015. Those lost vacation days represent roughly $61.4 billion in forfeited benefits, with Americans, in effect, volunteering hundreds of millions of days of free work to their employers, according to the study.

One of the main barriers to taking time off was a fear that the employee would return to a mountain of work (with 37% of respondents citing this as a motivation to stay in the office); 30% of those surveyed believe no one else in the workplace can do their job. Many of those surveyed – 80% – say they would be more likely to take vacation time if they felt fully supported and encouraged by their boss.

The online survey was taken by 5,641 people who work more than 35 hours a week and get paid time off. It was conducted by GfK and based on a representative random sample of the population. Economic analysis firm Oxford Economics used the survey results and data from the Bureau of Labor Statistics’ Current Population Survey to estimate historical levels of vacation activity.