Americans are leaving them on the table like never before when it comes to vacation days, essentially working for free almost one week per year, according to a new study by Oxford Economics.

U.S. workers are using only 77 percent of their paid time off, according to the research group's report released Tuesday. And the decline is not just tied to recent economic worries; use of vacation days are at their lowest point in the past four decades. In 2013, U.S. workers took an average of 16 days of vacation compared with 20.3 days in 2000, according to the report.