Call it the Lost Week. For decades, Americans took an average of 20 vacation days each year. They piled into the family wagon or hopped on a plane or train to get away from it all. It wasn't always as relaxing as expected, but most everyone returned home happier and ready for the next challenge. Exactly the point of a vacation.

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Then, beginning in 2000, the number of vacation days American workers took each year began ticking down. And down. And down. By 2015, Americans were using an average of 16 vacation days a year — nearly a full workweek less than the long-term average.

The kicker is that this Lost Week wasn't stolen from American workers; most still earned it from their employers. They just walked away from it, opting instead to essentially work for free for their employers one week a year.

Economic conditions don't explain the Lost Week. Researchers at the U.S. Travel Association (USTA) have found no correlation between unemployment rates and vacationing habits. Nor is there any indication that people take more vacation when consumer confidence is up.

Where researchers do find connections, though, is with the spread of technology. As the internet and smartphone technology took off, people started taking less time off. Even when workers did hit the open road, technology tethered them to the office like never before. A recent AARP Travel survey of boomers found that a third of them did some work while on vacation, with 4 out of 10 saying it was "somewhat" to "extremely" important to do so.

Workers offer up plenty of reasons why they're checking email beside a hotel pool or avoiding vacation altogether. The USTA survey found that workers were leery of returning to mountains of work. Some also feared that their supervisors might look askance at their taking time off.

"We are living in a world where the time demands [on] workers have risen exponentially," says Kathleen Gerson, a sociologist at New York University and author of The Unfinished Revolution: Coming of Age in a New Era of Gender, Work & Family.

"As jobs become more insecure, the need, desire or pressure to prove oneself through time at work increases."