Millions of women in the U.S. are shift workers, following schedules that fall outside the typical 9 to 5, yet little is known about how their hours may affect their reproductive health and fertility.
Now, a startling new review of prior studies suggests that shift work can take a serious toll.
According to research presented at the annual meeting of the European Society of Human Reproduction and Embryology in London this week, women who work irregular shifts are more likely to experience reduced fertility and greater menstrual disruption. Women who work nights specifically may have an increased risk of miscarriage.
"Anybody who works shifts has changes to their biological functioning, and it could be any one of a number of things that has an impact on [women's] ability to reproduce," Dr. Linden Stocker, a researcher with the University of Southampton in the U.K., told The Huffington Post.
Stocker and her colleagues reviewed 14 studies published between 1969 and 2013 that investigated the links between shift work -- which they defined as anything outside 8 a.m. and 6 p.m. -- and miscarriage, menstrual disruption (defined as cycles of less than 24 days and cycles of 32 or more days), and failure to become pregnant after one year of trying. All together, the studies included nearly 120,000 women.
Working a rotating shift was linked to a 33 percent greater rate of menstrual disruption and an 80 percent greater rate of sub-fertility.
Working nights in particular was not tied to menstrual issues or problems becoming pregnant, but it was linked to a greater risk of miscarriage.
The researchers describe the findings as novel, but warn that they should not be oversold. "We haven't proven causation; we've found an association," cautioned Stocker.
Women who work irregular shifts or nights may engage in any number of behaviors that could be influencing their reproductive outcomes. For example, working odd hours is often linked to sleep loss, decreased exercise and poorer diet.
But a growing body of research suggests that physiological changes accompanying shift work may be to blame and that any reproductive issues are a direct result of disrupted circadian rhythms.
In a preliminary study published last year, researchers manipulated the patterns of light and darkness to which mice were exposed in order to test what effect, if any, this would have on their fertility. A group of mice subjected to repeated changes in their light-dark cycle had a pregnancy success rate of only 22 percent, compared to a 90 percent success rate among a group of mice exposed to a typical day-night pattern.
"We were surprised at how dramatic the effect of manipulating the light-dark cycle was," Keith Summa, a researcher with Northwestern University and an author on that study, said in a statement. "We expected a negative effect from the circadian clock disruption, but not this much."
Some estimates suggest that as many as 15 million Americans do regular shift work, including more and more women. According to the Sloan Work and Family Research Network at Boston College, nearly one-third of companies with extended hours reported an increase in the number of female shift workers in a 2004 study. Doctors and nurses often work irregular hours, as do pilots, police officers, firefighters, restaurant and retail employees, home health aides and others.
Stocker said that women who do shift work should not stress about the new findings, particularly until future studies have replicated the results.
"We know that many things can help improve reproductive outcomes -- a well-balanced diet, exercise and looking after yourself generally," she said. "Whilst we wait for future studies to prove or disprove our findings, it's important that women focus on their overall health."