Americans work an average of roughly 1,836 hours each year, up nearly ten percent from 1,687 in 1979. Labor economists expect this number to keep growing, especially in competitive fields with high pay.

That's partially because technology can now shoot midnight emails straight to workers’ wrists and Silicon Valley juggernauts feed employees organic meals at work before shuttling them home on WiFi-enabled buses.

When work extends around the clock, everyone suffers. People lose sleep. They fall ill. They miss the Little League games and piano recitals. They burn out.

The problem tends to be amplified, however, for working mothers, according to a summary of research from the Harvard Business School's new Gender Initiative.

A working mother is more likely to judge herself harshly for spending time away from her family. Her colleagues are more likely to judge her, too -- for working long hours or leaving the office earlier.

Researchers found when a female employee clocks out before the work-culturally acceptable time, her colleagues are more likely to think: She's probably off to pick up her kids. If a male employee checks out early, they may think: He's off to meet clients.

Bosses, meanwhile, are more likely to dole out travel assignments to men, figuring women would rather not part with their families.

These assumptions can unfairly block skilled workers from advancing, said Robin Ely, a Harvard Business School professor who studies gender equity in the workplace. And they may help explain why women are promoted much less often than men to top management roles in practically every profession.

"We have these strong cultural ideals we try to live up to," Ely said. "I think some of the worst judgement anybody can fear is that they're a bad parent. And that judgement comes down much harder on women than on men who work all the time."

Ely's team recently examined an international consulting firm with generous family-friendly benefits, where 90 percent of the partners were men and most employees worked at least 60 hours each week.

The firm, unnamed in the study, asked the researchers: How can we decrease the number of women who quit and boost the number in leadership roles? The researchers asked the firm: Well, why do you think women aren't getting promoted?

"Virtually all interviewees — men and women — reiterated some version of the 'work-family narrative' to explain why women quit or failed to make partner: the job requires extremely long hours; women's (but not men’s ) devotion to family impedes their ability to put in the requisite hours, and their careers suffer as a result," the academics wrote.

During the employee interviews, Ely said, a disturbing anxiety emerged among young female workers. They looked at female managers, who beat odds to ascend the upper ranks. They viewed them as competent leaders. But they quietly wondered something their male colleagues never mentioned: If I find similar success, will I be a bad parent?

Perception did not match reality, they found. Male employees at the firm reported about the same level of desire to tend to their families as female employees. They all expressed guilt about time spent away from home. "Devotion to family" knew no gender.

"The difference was that women were more likely to take advantage of reduced-hours policies... and working reduced hours damaged their prospects at the firm," according to the findings.

Ely's conclusion: Progressive family-friendly policies, like paid maternity leave and sick days, simply aren't enough to quash gender inequality in a 24-7 work culture.