All work and no vacations make millennials stressed and less productive. Flickr / Starlit Beaches While the American workforce becomes more transitory, with employees working at home or freelancers taking jobs from full-time employees, the concept of the vacation from work gets eroded.

A study conducted by Alamo Rent a Car on vacation habits found that 40 percent of all Americans who receive paid vacation benefits did not use all of their allotted time off in 2014. Nineteen percent said they did not use at least five days of their paid vacation time in 2014.

The study asked those who did not use all of their allotted time why they did not use it, and almost half said they were simply too busy at work to do so.

Then there are those people who work during their vacation, and Millennials are most “guilty’’ of that. Thirty-four percent of Millennials said they worked every day of their vacations. They also reported that the effect of that work load was a decrease in productivity when they got back from their vacation.

The study compared American employees who used all of their vacation time against those that did not and found that 54 percent of those who use all of their vacation time did not work during their time off while only 37 percent of those who did not use all of their time were able to unplug completely.

Some companies allow workers to carry over unused vacation time to the next year, but the Alamo study found that the carryover has an economic effect. Citing research by Oxford Economics in 2014, U.S. companies carried forward $65.6 billion in accrued time off costs, which averaged to a cost per employee of $1,898. Companies with more than 500 employees who allow time off to be carried over paid $2,609 per employee as a result of the carryover.

Who couldn't use an extra 1.6 days off work? REUTERS/Cathal McNaughton Among employees who are not allowed to carry over their paid time off, the average worker left unused 1.6 days in 2013.

The Oxford study found that only 77 percent of all paid vacation time was actually taken by employees in 2013.

Multiple studies show that productivity is negatively affected when an employee does not fully release from work during vacation. The HR consulting firm Randstad studied the issue of working during vacation and found that 40 percent of Millennials feel guilty about using their vacation time, while only 18 percent of Baby Boomers feel the same way.

“There’s an impact on productivity, there’s an impact on allowing people to release and come back refreshed,” said Randstad chief HR officer Jim Link in an interview with Forbes.