That is the finding of a University of Michigan study providing the first known nationally representative data documenting substantial gender differences in getting up at night, mainly with babies and small children.

And women are not only more likely to get up at night to care for others, their sleep interruptions last longer---an average of 44 minutes for women, compared to about 30 minutes for men.

"Interrupted sleep is a burden borne disproportionately by women," said sociologist Sarah Burgard, a researcher at the U-M Institute for Social Research (ISR). "And this burden may not only affect the health and well-being of women, but also contribute to continuing gender inequality in earnings and career advancement."

The study, forthcoming in the peer-reviewed journal Social Forces, was funded by the National Institute of Child Health and Human Development, the National Institute on Aging, and the Sloan Foundation.

For the study, Burgard analyzed time-diary data from approximately 20,000 working parents from 2003 to 2007, drawn from the U.S. Census Bureau's American Time Use Survey.

The gender gap in sleep interruptions was greatest during the prime childbearing and child-rearing years of the twenties and thirties, she found.

Among dual-career couples with a child under the age of one, 32 percent of women reported sleep interruptions to take care of the baby, compared with just 11 percent of men. The proportion reporting interrupted sleep declined with the age of the child, with 10 percent of working mothers and 2 percent of working fathers with children ages 1 to 2 reporting sleep interruptions, and just 3 percent of working mothers and 1 percent of working fathers with children ages 3 to 5.

"What is really surprising," Burgard said, "is that gender differences in night-time caregiving remain even after adjusting for the employment status, income and education levels of each parent. Among parents of infants who are the sole breadwinner in a couple, for example, 28 percent of women who are the sole breadwinner report getting up at night to take care of their children, compared to just 4 percent of men who are the only earner in the couple."

In related research, Burgard and colleagues found that women get slightly more sleep compared to men. But getting about 15 minutes more total sleep a day may or may not compensate for the greater sleep interruptions women face.

"Women face greater fragmentation and lower quality of sleep at a crucial stage in their careers," Burgard said. "The prime childbearing years are also the time when earnings trajectories are being established, and career advancement opportunities could well be foregone if women reduce their paid work time or see their workplace performance affected because of exhaustion. As a result, sleep interruption may represent an under-recognized 'motherhood penalty' that influences life chances and well-being."

The findings also have implications for public health interventions to improve sleep. "Generally, these interventions target individual behaviors, such as the use of alcohol, caffeine or tobacco," said Burgard. "Or they focus on nightly routines that help people to relax and fall asleep or stay asleep more successfully.

"But for parents of young children, the best approach might be discussions and negotiations about whose turn it is to get up with the baby tonight."

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Established in 1949, the University of Michigan Institute for Social Research (ISR) is the world's largest academic social science survey and research organization, and a world leader in developing and applying social science methodology, and in educating researchers and students from around the world. ISR conducts some of the most widely-cited studies in the nation, including the Thomson Reuters/University of Michigan Surveys of Consumers, the American National Election Studies, the Monitoring the Future Study, the Panel Study of Income Dynamics, the Health and Retirement Study, the Columbia County Longitudinal Study and the National Survey of Black Americans. ISR researchers also collaborate with social scientists in more than 60 nations on the World Values Surveys and other projects, and the Institute has established formal ties with universities in Poland, China, and South Africa. ISR is also home to the Inter-University Consortium for Political and Social Research (ICPSR), the world's largest digital social science data archive.