Rotating shift work may affect menstrual cycle

NEW YORK (Reuters Health) - Women who work rotating shifts may be somewhat more likely to experience shifting menstrual cycles according to a new study that raises the possibility of work schedules affecting fertility.

In a study of more than 71,000 U.S. nurses, researchers found that those working rotating shifts were more likely than other nurses to have irregular menstrual periods.

Irregular, for the purposes of the study, meant that the time between a woman’s periods usually varied by more than a week.

Women on rotating shifts were also more likely to have either very short menstrual cycles (fewer than 21 days between periods) or very long ones (40-plus days) -- although few women in the study were at either of those extremes.

In general, menstrual irregularities make it harder for a woman to become pregnant. Whether shift work induces disruptions in some women’s cycles that contribute to infertility remains unknown for now.

The current findings do not actually prove that shift work, itself, disrupts women’s menstrual periods, according to lead researcher Christina C. Lawson, of the U.S. National Institute for Occupational Safety and Health.

Her team factored in a number of other things that might explain the link, however -- like the women’s age, weight and exercise levels -- and shift work was still connected to menstrual irregularities, Lawson told Reuters Health.

“That gives us more confidence in the association,” Lawson said, but an association does not necessarily equate to cause-and-effect.

On the other hand, there are physiological reasons to believe that rotating shifts could alter a woman’s menstrual cycle.

Working nights disrupts the body’s natural circadian rhythms, and studies have shown that this can alter basic physiological functions -- like blood pressure control and hormone production.

“We don’t really know the exact mechanism,” Lawson said. “One possibility could be that exposure to light at night alters melatonin production.”

Melatonin is a hormone produced mainly during dark hours that helps regulate sleep and other body processes. Its relationship to reproductive hormones in humans is not clear.

The study findings, published in the journal Epidemiology, are based on data from a long-running study of female nurses from across the U.S.

At the start of that study, just over 5,000 women between the ages of 28 and 45 had worked at least 20 months of rotating shifts in the past 2 years. (A rotating shift meant any month where a woman worked at least 3 nights in addition to days and evenings. The study did not look at women who worked nights only.)

Of women who did the most rotating shifts, 12 percent said they had irregular periods. That compared with 9 percent of the nearly 58,000 women who had worked no rotating shifts in the past 2 years.

When Lawson’s team accounted for other factors, women who’d worked at least 20 months of rotating shifts were 23 percent more likely to have irregular periods than those who’d worked none.

Women who’d worked fewer rotating shifts fell somewhere in between.

When it came to having very short or long menstrual cycles, the odds were higher among nurses who’d worked the most rotating shifts.

Few women were at those two extremes, though: 2 percent of those with at least 20 rotating shifts said their menstrual cycles lasted 40 to 50 days, for example. That compared with 1 percent of all other women.

For women who must work the night shift, Lawson said, “my biggest advice is to try to take care of yourself and catch up on your sleep when you can.”

It’s not clear whether that catch-up sleep can right any menstrual irregularities. But it’s a wise move for your overall well-being anyway, according to Lawson.

She also suggested that women who work nights pay close attention to their diet and exercise habits -- both of which can be challenging for people on irregular work schedules.

In this study, Lawson noted, overweight and obese women were more likely than normal-weight women to have irregular menstrual cycles.

SOURCE: Epidemiology, May 2011.