According to a study presented at the 25th European Congress of Endocrinology, it takes 4 weeks of a shift work pattern before the biological clock of the women is thrown off balance, reducing their chances of conceiving.

In the study, the researchers from the Institute of Cellular and Integrative Neurosciences (INCI) and the University of Strasbourg mimicked long-term shift work conditions in female test models by constantly shifting the light-dark cycle. They also delayed and advanced the exposure duration to light by 10 hours for the study duration of 4 weeks.

The results of the study showed that changes in the biological clock led to the inhibition of luteinizing hormone, a hormone that triggers ovulation. This in turn led to a reduction in the fertility of the female test models.

Hence, the researchers concluded that four weeks of chronic shift exposure impairs the transmission of light information. As a result, the connection between the master biological clock and kisspeptin neurons, the neurons that signal the surge of pre-ovulatory luteinizing hormone, is weakened. 

(Source: https://theprint.in/health/research-explains-how-shift-work-may-influence-fertility/1578205/ )