American workers are definitely time wasters — they waste millions of hours of paid time off every year because they're too busy working to use the PTO they've accumulated.

In its second survey in four months, the U.S. Travel Association attempted to quantify the “cost” to employees of not taking their PTO. An earlier survey revealed the reasons behind the tendency not to use all of one's PTO.

This time around, here are the numbers that emerged from the data-crunching:

• American workers permanently lost a total of 169 million days of paid time off across the workforce in 2013;
• American employees give up $52.4 billion in benefits by not taking their full PTO;
• The average cost per employee is $504;
• U.S. employees took an average of 16 vacation days in 2013 compared to an average of 20.3 days as recently as 2000.
• U.S. employees who left 11-15 days unused are less likely (by 6.5 percent) to receive a raise or bonus than those who used all of their vacation days;

Those who leave 11 days or more unused reported being “very” or “extremely” stressed with their work lives, compared to those who left less time on the table or took all their PTO (31 percent vs. 25 percent).