Yet another new report confirms what we already know about Americans and vacation days: they don't take enough.

In 2013, employees took an average of 16 days off. This compares to the 20.3 vacation days Americans used on average each year from 1976 to 2000. The report was conducted by the advisory firm Oxford Economics for the U.S. Travel Association. 

Oxford Economics analyzed data from the U.S. Bureau of Labor Statistics in conjunction with a June 2014 survey of 1,303 American workers conducted by GfK Public Affairs and Corporate Communications.

By forfeiting paid time off, workers provide free labor for their employers at an average of $504 per employee, the report said. 

So why are employees consciously choosing to lose out? 

Joe Robinson, a work-life trainer and speaker, suggested that workers have been afraid to take vacation days in the recent era of layoffs. "It's called defensive overworking. They work long hours and skip vacations to insulate themselves from cutbacks," he told CNN.

But it's futile, he said. "People who don't take their vacations get laid off just like everyone else."

Another reason employees avoid taking vacation is because they feel trapped under a pile of work. Companies today are getting by with leaner staffs, meaning more people are doing the work of several people, Robinson said. This makes it hard to escape.

Though this paints a pretty bleak picture of Americans' work-life balance, several companies have recently implemented policies aimed at reversing the trend. For example, Virgin Group founder Richard Branson announced that employees on his staff can take off as much time as they like--no questions asked. 

U.S. Travel Association CEO Roger Dow also told CNN that some companies have started giving bonuses to employees who use their entire allotment of paid leave.

Dow suggested that these companies have been influenced by a growing body of research, which indicates that in the long-run, more time off increases productivity and is better for the bottom line.