Women who work shifts are more likely to have reduced fertility compared with women who work regular hours, according to a new analysis of data on the effect of shift work on health.

Women who work shifts are also more likely to suffer disrupted periods, while working nights is linked to higher rate of miscarriage, says Dr Linden Stocker from the University of Southampton, UK, who presented the study at the annual meeting of the European Society of Human Reproduction and Embryology (ESHRE 2013) in London on Tuesday.

Previous studies have looked at the effect of shift work, which encourages people to be active outside of their normal sleep-wake cycle and is linked to sleep deprivation, on heart risk, social support, stress, unhealthy behaviors (such as smoking, diet, alcohol, lack of exercise) and metabolic changes (higher blood pressure and cholesterol).

Some of these studies have found shift work is linked to these factors, but the results tend to be varied and the designs inconsistent so it is not easy to draw clear conclusions.

Also, very little has been published on the effects of shift work on reproductive health and fertility.

This new study, a meta-analysis that pooled data and applied a consistent design, reviewed research published between 1969 and January 2013 that looked at the effect of shift work on health.

Dr. Stocker says they compared the effect of shift working (alternating shifts, evenings and nights) and non-shift working in women, and pulled out results relating to fertility, miscarriage rates and menstrual dysregulation.

The analysis, which covered nearly 120,000 women, found those working shifts had an 80% higher rate of fertility problems, and a 33% higher rate of disrupted menstrual periods compared to women working regular hours.

Women working only nights did not show a statistically significant higher risk of disrupted periods or difficulty in getting pregnant, but they did show a 29% higher rate of miscarriage.

However, the higher rate of miscarriage was not apparent in women who worked nights as part of a shift pattern.

The team describes these results as “novel” but consistent with other studies, which have tended to find problems later in pregnancy.

Dr. Stocker is keen to point out that while they have shown a link between shift work and problems with fertility and reproductive health in women, they have not shown that one is caused by the other. Dr. Stocker says:

In humans, the long-term effects of altering circadian rhythms are inherently difficult to study. As a proxy measure, the sleep disruption demonstrated by the 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 other studies can now repeat these findings, then they “have implications for women attempting to become pregnant, as well as for their employers”, say the researchers.

It could be that more friendly shift patterns is the answer, although it is not clear which shift pattern is most beneficial for those wishing to conceive.

Dr. Stocker also points out that only some aspects of reproductive health were affected by shift work: the underlying biology is complex and “not the same across all the disease processes”.

“Indeed, it is probable that completely different causes underlie menstrual dysfunction, miscarriage and subfertility. This may explain why the effects of different types of shift work are seen in some groups of women, but not others,” Dr. Stocker explains.

One possible explanation she offers is that disrupting the 24-hour sleep-wake cycle (the circadian rhythm) may upset the body’s “clock genes”, which are linked to biological changes.

The International Agency for Research on Cancer (IARC) has classified night work as a “probable human carcinogen”, and suggests the likely culprit is the disruption of human circadian rhythms.

Written by Catharine Paddock PhD