Tis the season to be jolly, bright and chained to your cubicle.

A quarter of full-time workers ages 18 to 25 who get paid time off say that by year’s end they won’t have taken any of their paid vacation days, according to a new survey.

The study, which Princeton Survey Research Associates conducted on behalf of personal finance website Bankrate Inc., surveyed more than 1,000 full-time American workers, with a focus on those who receive paid vacation time from their employers.

Older baby boomers between the ages of 62 and 70 are most likely to use up all their vacation days, followed by Generation Xers, or workers between 36 and 51. Young millennials, 18 to 25, are least likely to take all their days. Only 35 percent of them said they would max out their vacation time.

On average, millennial workers, aged 18-34, are leaving 21 days unused this year, and boomers ages 52 to 70 are leaving behind 25. Gen Xers are leaving just 13 days unused.

There is a reason millennials and boomers have similar vacation strategies, said Arielle O’Shea, the investing and retirement specialist for personal finance website NerdWallet: They are trying to prove themselves in competitive fields and concerned they will be looked down on because of their age.

They might also just not be able to afford vacations, millennials because they are comparatively poor and paying off student loans, and boomers because they are saving for retirement (and maybe paying their kids’ student loans).

But many say they are just too busy working: 28 percent of millennials and 25 percent of boomers said they weren’t using up their vacation days because they had too much work to do. Just 14 percent of Gen Xers, who are right in the middle of parenting years, said the same. (Gen Xers were also most likely to decide not to use a vacation day in 2016 to roll it over to next year instead.)

And who kept on working just because they really, really love their jobs? That would be young millennials. Nearly a third said their love of their jobs kept them from taking time off.

Still, the study found, workers across all generations prefer cash to paid time off. Asked to choose between having an extra week of paid vacation time or an extra week’s worth of pay, workers overwhelmingly picked cash.

Says O’Shea: “Vacations are expensive, and nothing beats cash.”