Americans did not use 658 million vacation days last year. For the first time ever, more than half of Americans (55%) did not use all of their days off, according to a study done by Project: Time Off. We are becoming the “No Vacation Nation.”

While Americans used to average three weeks of vacation a year in 2000, in 2015 they only took 16.2 days. That represents a loss of almost one week in 15 years.

Why would people essentially volunteer a week of their time every year for their company? The two biggest factors cited in the study were fear they would return to a mountain of work (37%) and that no one else can do their job (30%).

Unlike other developed countries, in the U.S. employers are not required to give employees paid time off. Employees in the European Union get a minimum of 20 days a year.

While a business owner or CEO may appreciate that their employees didn’t take their allotted time off, research shows their productivity may actually be lower when they don’t take breaks.

Studies show that when employees take time off, their productivity increases. “There is a lot of research that says we have a limited pool of cognitive resources. When you are constantly draining your resources, you are not being as productive as you can be. If you get depleted, we see performance decline. You’re able to persist less and have trouble solving tasks,” said Allison Gabriel, an assistant professor of management at Virginia Commonwealth University in the article “The Secret to Increased Productivity: Taking Time Off.”

In a Wall Street Journal blog, Dr. Kathleen Potempa wrote, “In addition to mental and physical stressors, long periods of work without vacation can lead to reduced productivity, diminished creativity, and strained relationships. Americans seem to believe that logging more hours leads to increased output, but respite deprivation can actually increase mistakes and workplace animosity—in addition to prompting or exacerbating stress-related illnesses.”

CEOs and business owners should look at their own calendars and clear time for vacation as well. Reed Hastings, the CEO of Netflix, takes six weeks a year. “I take a lot of vacation and I’m hoping that certainly sets an example. It is helpful. You often do your best thinking when you’re off hiking in some mountain or something. You get a different perspective on things.”

COO of Facebook Sheryl Sandberg says she was able to write her best-selling book “Lean In: Women, Work and the Will to Lead” because she took all of her vacation days. (I’ve written a book, and would be on the side of people who argue that’s not quite what I’d call a vacation.)

Tony Schwartz, the president/CEO of The Energy Project and author of “Be Excellent at Anything” says at The Energy Project they teach “the greater the performance demand, the greater the need for recovery.” Feeling burnt out one year, he went on vacation and completely disconnected from digital distractions. “By the end of nine days, I felt empowered and enriched. With my brain quieter, I was able to take back control of my attention. In the process, I rediscovered some deeper part of myself.”

Mark Douglas, CEO of the marketing and advertising company SteelHouse, recognized the need for his employees to take vacation and offered them unlimited vacation when he founded the company in 2010. But perhaps due to the reasons stated above, people weren’t taking much.

So he decided to pay them. To take vacation. He pays his employees $2,000 a year to go anywhere in the world. They can split up the money for more than one trip, or use it all at once. Employees who request the money in the form of a bonus are turned down. They must spend it on a vacation.

As a result, his turnover rate is extremely low. Out of 250 employees, only five people left the company in a three-year period, with three of them leaving for reasons unrelated to the job.

So if you are feeling a bit anxious when you see all the empty desks and email vacation notices at your company over the holidays, think of it this way: they are recharging their batteries and will come back more productive than ever.

Take some time off yourself. And enjoy.

My book “How Not to Hire a Guy Like Me: Lessons Learned from CEOs’ Mistakes,” is now available as an ebook.