Economist Casey Mulligan says that one reason for the discrepancy between male and female wages is that men are more willing to work the night shift:
The vast majority of workers perceive work from 9 a.m. to 5 p.m. to be more desirable than work during the off-hours, and many of the off-hours workers are compensated with higher pay for the less desirable schedule. A variety of factors — including, some economists and many women’s rights advocates say, gender discrimination — may cause women to be paid less than men, but part of the reason may be the hours they choose to work.
The night shift story is part of the same basic pattern: women are less willing or able than men to put in hours at work that are incompatible with family life. Working 60 hours a week to get ahead is very difficult for mothers, and so is working the night shift. The nub of the issue is that we live in a society where men often feel comfortable, or at least justified, working those sorts of hours even if they're parents, while women don't.
Describing this as the "hours they choose to work" is somewhat misleading. Social pressures can shape people's choices. A mother might prefer to work long hours and have the husband pick up the slack at home, but the husband might not be willing. Or perhaps the husband is willing but neither of them wants to be have their friends and family constantly wondering about the non-traditional relationship. Or maybe they are willing, but their children expect to see the mother more because their friends see their mothers more, and the mother can't take the guilt of explaining to her kids why she has to see them less than other mommies do.