In a study of more than 71,000 US 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.
The researchers, led by Christina C. Lawson, of the US National Institute for Occupational Safety and Health, adjusted for a number of other things that might explain the link - like the women's age, weight and exercise levels. But shift work was still connected to menstrual irregularities, Lawson said.
"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.
"We don't really know the exact mechanism," Lawson said. "One possibility could be that exposure to light at night alters melatonin production."
The study findings, published in Epidemiology, are based on data from a long-running study of female nurses from across the US.
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 two years. (A rotating shift meant any month where a woman worked at least three 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% said they had irregular periods. That compared with 9% of the nearly 58,000 women who had worked no rotating shifts in the past two years.
Shift work, irregular periods related
When Lawson's team accounted for other factors, women who'd worked at least 20 months of rotating shifts were 23% 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% of those with at least 20 rotating shifts said their menstrual cycles lasted 40 to 50 days, for example. That compared with 1% of all other women.
Catch up on sleep
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.
In this study, Lawson noted, overweight and obese women were more likely than normal-weight women to have irregular menstrual cycles. - (Amy Norton/Reuters Health, April 2011)
Let there be darkness