A fear of “returning to a mountain of work” is the top factor keeping Americans chained to their desks and it comes at a cost. Employees who forfeit vacation time are less likely to have been promoted within the last year and to have received a raise or bonus in the last three years compared to workers who took all of their PTO.

But beyond that main take-away, the study revealed two other, more surprising trends.

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First, Americans showed early signs of reversing their bad vacation habits. The share of employees who failed to use all their vacation time was actually down one percentage point from 55% in 2015. And the average number of vacation days logged by employees—16.8—was up from 16.2 in 2015 and an all-time low of 16 in 2014 and 2013.

Second, the positive trend was driven largely by male employees, with 48% of men—up three percentage points from 2015—taking all their vacation time in 2016, compared to 44% of women. That finding is especially striking since women are more likely than men—58% versus 49%—to say vacation time is “extremely” important to them.

That is a statement of idealism, not behavior, the study says:

High stress, guilt, and workload concerns may be keeping women from using their time off. Women report experiencing more stress than men at home (48% to 40%) and at work (74% to 67%). They are also more likely to say that guilt (25% to 20%) and the mountain of work they would return to (46% to 40%) hold them back from taking time off. Women also worry more than men about vacation making them seem less committed to their job (28% to 25%)

The gender divide was even starker among millennials, with 51% of millennial men using all of their vacation time in 2016, compared to 44% of their female counterparts.

Katie Denis, senior director of Project Time Off who authored the report, says the reasons for the gap are difficult to pinpoint, but one factor could be millennial men’s growing professional confidence; they feel secure enough in their jobs to be out of the office.

Millennial women, meanwhile, are less likely “to vocalize” needing a vacation, Denis says. “They feel like they need to apologize” for taking time off, she says.

The study says a lack of communication from managers has “created a vacuum” where negative perceptions about taking time off thrive. While managers agree that vacation improves health and well-being (82%), boosts morale (82%), and alleviates burnout (81%), they fail to express that outlook explicitly. Two-thirds of employees surveyed said their “company culture is ambivalent, discouraging, or sends mixed messages about time off,” a share that’s virtually unchanged since 2014.

Millennial women were even more likely to feel that way. Their managers, Denis says, need to “create the avenues to say [taking a vacation] is OK.”