In one of Back to the Future’s pivotal scenes, Marty McFly strums “Earth Angel” as he watches his future parents’ relationship—and his own existence—dissolve from the stage. I’ve always found this scene terrifying.

And when I think of that scene today, with the McFly kids fading away, those kids might as well be vacation days. If we were to rev up the DeLorean, our vacation days would be there. We used to take a lot more days off—more than 20, historically. But these days, we’re down to less than 17 days on average.

Vacation use is up slightly, but American workers gave up 662 million days last year alone. And we’re losing a lot more than a tan, particularly for the youngest generation in the workforce. Project: Time Off research shows that Millennials are not only not taking the vacation they earn, they are the generation that is most likely to forfeit time off, even though they receive the least number of vacation days.

Doesn’t sound like the Millennial stereotypes you’ve heard? Millennials are afraid, not entitled. Compared to Boomers, Millennials are at least twice as likely to say they are fearful of losing their job. This cohort worries about what the boss might think, wants to show complete dedication, and does not want their bosses to see them as replaceable.

Millennials make up half of the workforce, providing them with the numbers to make a serious, long-term impact on American culture. About a quarter (and counting) of Millennials are in management roles. As managers, they are much more likely to have denied vacation requests due to work pressure than any other generation.

What happens to America’s work culture when a generation that prizes work martyrdom more than any other has the majority? And what does it mean for Generation Z as they approach working age? The vacation trendline is concerning today—but it could be poised to grow worse if we do not change.

We already do not prioritize time off the way we should and, despite all the evidence to the contrary, the myth that skipping vacation will help employees get ahead continues to thrive—and perhaps even strengthens—with every generation. (While we’re here: our data proves that employees who forfeit vacation time are less likely to get raises, bonuses, and promotions.)

We need to care about taking the time to explore the world with the same fervor we care about face time at work. No one disputes the importance of vacation, but too few consider the consequences of time off becoming the innocent bystander of an insane work culture. The effects of letting travel fall into the category of “maybe someday” are deeper and broader than work culture—they have profound ripple effects.

Mark Twain said, “Travel is fatal to prejudice, bigotry, and narrow-mindedness, and many of our people need it sorely on these accounts. Broad, wholesome, charitable views of men and things cannot be acquired by vegetating in one little corner of the earth all one’s lifetime.”

If we drop time off from the priority list, we lose the chance to understand other cultures—and really other people—better, whether they’re in Paris, France or Paris, Texas. These days, building those bridges of understanding feels particularly important. We are a nation and a world with deep divisions, divisions that could be lessened by meeting each other face to face.

Americans still take time off, but not the way we used to. It’s time to go back…for the future.  

Katie Denis leads the Project: Time Off initiative, a national movement to transform American attitudes and change behavior around vacation time. Through rigorous research and focused communications, Project: Time Off aims to shift culture so that taking time off is understood as essential to personal wellbeing, professional engagement, business performance, and economic expansion.

You can read more from Katie here.