London, 9 July 2013: Shift work, which encourages sleep deprivation and patterns of activityoutside the circadian rhythm, has been associated with a greater risk of ill health and loss ofwell-being in some (but not all) studies.(1) However, little is known about the effects of shiftwork on reproductive health and fertility.
Now, a study reported today at the annual meeting of ESHRE, by Dr Linden Stocker from theUniversity of Southampton, UK, indicates that working shift patterns is associated with anincreased risk of menstrual disruption and subfertility.
The study is a meta-analysis of all studies on the subject published between 1969 and January2013. It compares the impact of non-standard working schedules (including night-shift work andmixed-shifts) with that in women not working shifts. The end-points were early reproductiveoutcome parameters, including menstrual dysregulation, female fertility and miscarriage rates.
The study, which included data on 119,345 women, found that those working shifts (alternatingshifts, evenings and nights) had a 33% higher rate of menstrual disruption than those workingregular hours (odds ratio 1.22, statistically significant) and an 80% increased rate of subfertility(OR 1.80, statistically significant).(2)
Women who worked only nights did not have a statistically increased risk of menstrualdisruption or difficulty conceiving, but they did have an increased rate of miscarriage (OR 1.29),although this increased risk of miscarriage was not observed in women who worked nights aspart of a shift pattern.
The investigators describe their findings as "novel", but in keeping with other studies (whichfound adverse effects in later pregnancy). "If replicated," they said, " our findings haveimplications for women attempting to become pregnant, as well as for their employers".
On the implications of the study Dr Stocker said: "Whilst we have demonstrated an associationbetween shift work and negative early reproductive outcomes, we have not proven causation. Inhumans, the long-term effects of altering circadian rhythms are inherently difficult to study. As aproxy measure, the sleep disruption demonstrated by the shift workers in our study createsshort- and long-term biological disturbances. Shift workers adopt poor sleep hygiene, suffersleep deprivation and develop activity levels that are out-of-sync with their body clock.
"However, if our results are confirmed by other studies, there may be implications forshift workers and their reproductive plans. More friendly shift patterns with lessimpact on circadian rhythm could be adopted where practical - although the optimalshift pattern required to maximise reproductive potential is yet to be established."
In noting that only some reproductive outcomes were affected by shift work, Dr Stockerreported that the underlying biological disturbances involved in reproductive difficulties "arecomplex and not the same across all the disease processes". "Indeed," she said, "it is probablethat 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 ofwomen, but not others."
She added that one possible explanation for the overall findings is that the disruption ofcircadian rhythm can influence the biological function of "clock genes", which have been shownto be associated with changes in biological functions.
Source: European Society of Human Reproduction and Embryology