Our circadian rhythm, also known as our biological clocks, plays a huge part in our health. Many studies have shown that sleep deprivation, or other activities that are outside of the rhythm, can lead to health-related problems, including working at other times than a typical day shift. A study has found that doing this might actually cause women to be at a greater risk of suffering fertility and reproductive health problems.

The study was reported by Dr. Linden Stocker, of the University of Southampton, UK, at the annual meeting of the European Society of Human Reproduction and Embryology. It was an analysis of all studies on the subject between 1969 and January 2013, and compared the reproductive health of 119,345 women who worked night- and mixed-shifts to women who worked normal shifts. The end-points for these women were negative early reproductive outcomes, including menstrual dysregulation, female fertility, and miscarriage rates.

The researchers found that women who worked alternating shifts, evenings, or nights, had a 33 percent higher rate of menstrual disruption than those working normal hours. They also found that these women were 80 percent more likely to be less fertile than normal. While women who worked only nights didn't show any signs of menstrual disruption, they had a higher rate of miscarriage.

The researchers described this study as "novel," saying that the study could 'have implications for women attempting to become pregnant, as well as for their employers."

"Whilst we have demonstrated an association between shift work and negative early reproductive outcomes, we have not proven causation," Dr. Stocker said in a press release. "In humans, the long-term effects of altering circadian rhythms are inherently difficult to study. As a proxy measure, the sleep disruption demonstrated by shift workers in our study creates short- and long-term biological disturbances.

"Shift workers adopt poor sleep hygiene, suffer sleep deprivation, and develop activity levels that are out-of-sync with their body clock."

If their results can be confirmed with other studies, then it might have implications for women who work nights, but are planning on having a child.

"More friendly shift patterns with less impact on circadian rhythm could be adopted where practical - although the optimal shift pattern required to maximize reproductive potential is yet to be established," Dr. Stocker said.

She also noted that it's probable "that completely different causes underlie menstrual dysfunction, miscarriage, and subfertility," since working odd shifts only affected some reproductive outcomes.

But working night shifts has also been associated with an increased risk of type 2 diabetes, ovarian cancer, and breast cancer. A study published last week found that women who worked the night shift for 30 years or more were more likely to develop breast cancer.

For the study, researchers analyzed medical records of over 1,100 women who had developed breast cancer and 1,200 women who hadn't, over the course of five years. They compared these results to self-reported night shift employment questionnaires and found that about one-third of all the women had worked night shifts. These women were 50 percent more likely to develop breast cancer.

Source: Grundy A, Richardson H, Burstyn I, et al. Increased Risk Of Breast Cancer Associated With Long-Term Shift Work In Canada. Occupational Environmental Medicine. 2013.