Editor's note: This story was updated on August 5, 2019.

"Work martyrs" who don't use their available time off are leaving millions of vacation days unused, new research shows, which could increase employees' burnout and harm their health.

The Project: Time Off coalition, which tracks and analyzes Americans' vacation time, recently released a report detailing the vacation habits of U.S. workers. It found that in 2015, 55 percent of Americans did not use all of their vacation time, marking the first incidence of Project: Time Off ever reporting a majority of U.S. workers not taking advantage of all of their vacation time.

That amounts to 658 million unused vacation days, 222 million of which were lost without the possibility of being rolled over, paid out, or banked for any other benefit. American workers are, in effect, forfeiting $61.4 billion in benefits when they don't use their vacation days.

The use of vacation days has been on a steady decline among American workers. From 1978 to around 2000, full-time U.S. workers earned and took an average of 20 vacation days per year. By 2015, full-time workers earned almost 22 vacation days but only used about 16.

According to Project: Time Off's 2016 report, the most common reasons people leave their vacation days unused include:

• Fear of returning to a mountain of work (37 percent);
• Not having anyone else to do the job (30 percent);
• Inability to afford a vacation (30 percent);
• Difficulty taking time off with more growth in a company (28 percent);
• Desire to show complete dedication (22 percent); and
• Fear of being seen as replaceable (19 percent).

It's not always easy to leave the office, especially as technology has made it more challenging to completely disconnect from work. But experts say that time off is crucial for reenergizing the mind and body—and that ultimately it leads to increased workplace productivity.

"Vacation time reduces burnout and fatigue, and even blood pressure and strain go down," says Mina Westman, a professor of management at Tel Aviv University. "If you want to have a healthy workforce, physically and mentally, people need to have vacations. The body can't work nonstop."

"Happiness is actually the fast track to success," says Emma Seppala, science director of Stanford University's Center for Compassion and Altruism Research and Education. "If you take time off you're going to be more productive, you're going to have higher performance, you're going to have better relationships with people, be more charismatic—the list goes on" (Project: Time Off, State of American Vacation report, accessed 6/22/16; Samuelson, Washington Post, 6/19; Gutierrez, Kansas City Star, 6/16).

While on vacation—don't let your inbox get the best of you

So, what does it take to be an inbox ninja? Check out our infographic to get best practices for email-specific writing and email management. You'll also find tips for email formatting, scripting, and structure, plus approaches to collaboration and decision-making over email.