When you don't use your paid time off, it ends up costing you.

Not only are you more likely to be stressed and feel overworked, you lose out on the monetary value of those forfeited days — the ones that can't be rolled over or paid out.

In 2016, more than half of all Americans forfeited vacation days, amounting to 206 million unused vacation days worth some $66.4 billion, according to a survey of 7,331 full-time employees.

For the average worker, the lost value of that untaken paid time off comes out to $604, according to a report by Project: Time Off, which is sponsored by the U.S. Travel Association.

The good news is American workers are beginning to take at least a little more time off. After the 2008 financial crisis, the number of vacation days Americans took on average dropped from around 19 to 16. Last year, the number increased modestly for the second year in a row, to an average of 16.8 days.

But many professionals still worry about taking vacation days.

"Seventy percent of forfeiters believe that their company culture is negative, sends mixed messages, or is silent about vacation time," says Project: Time Off Senior Director Katie Denis in a statement.