In women, respecting the circadian rhythm probably has an impact on fertility, explains Valérie Simonneaux, research director at the CNRS at the Institute of Cellular and Integrative Neurosciences (INCI), during the International Endocrinology Days of June 2025. This is in any case what numerous studies carried out on mice tend to show, and what a clinical study in preparation aims to demonstrate in humans. In particular, working with shifts and irregular hours could affect female sex hormones and the risk of fertility disorders.
The LH surge, an indicator of menstrual cycle synchronization
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“ Almost all genes expressed in the ovaries have daily rhythms. The LH receptor gene, for example, is expressed so that there are a maximum number of receptors at the time of the LH surge.", illustrates Valérie Simonneaux. This surge of LH, or luteinizing hormone, is essential for inducing ovulation in the middle of the menstrual cycle. Triggered by the hypothalamus – a cerebral structure at the base of the brain – of all female mammals, this preovulatory peak always occurs at a time dictated by the circadian rhythm. More precisely, this peak occurs at the transition between the period of rest and activity, that is, the end of the night for women or at dusk for female mice, a nocturnal species. " To keep the body in sync with the environment, the neurons of the suprachiasmatic nuclei, managing the biological clock in the hypothalamus, are very sensitive to light detected by the retina", explains Valérie Simonneaux.
Repeatedly exposing the retina to light during the night therefore desynchronizes the clock and all the biological functions that result from it. In mice exposed to light several hours in advance, the LH surge is brought forward. Jet lag is created by moving the time of day forward a few hours. The peak will then take about two weeks to resynchronize once the initial rhythm is restored.", says Valérie Simonneaux. A time that corresponds to three ovarian cycles in mice.
Rotating shifts are most likely to affect female fertility
It is when exposure to light is irregular that the disturbances become major in female mice, with a near disappearance of the LH surge. Although ovulation itself could not be observed, the researcher reports a drop in fertility visible in particular by a delay and a reduction in the number of pregnancies. It is not a total abolition but a reduction in fertility", explains Valérie Simonneaux. Changing schedules, known as "rotating shifts," are therefore more disruptive in mice than staggered but constant schedules, she adds. In humans, women with rotating shifts, like hospital staff, would therefore potentially be more exposed to fertility disturbances. This is what the study by Valérie Simonneaux and the Strasbourg University Hospital aims to evaluate, which plans to monitor the LH peak of 30 to 40 women with normal or shift work schedules. It has never been shown yet, but some epidemiological studies have noted irregular, lengthened or shortened menstrual cycles and altered hormone concentrations.. »
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In addition to increasing age at conception or metabolic disturbances, the environment also has an influence on the decline in female fertility, says Valérie Simonneaux. Perhaps women trying to conceive should be advised to monitor light exposure and changing schedules.. » PMA techniques could also benefit from taking into account the schedule. These doctors always perform ovarian stimulation at the same time, but do not necessarily think about testing different times to optimize egg collection or inject sperm, for example.", concludes Valérie Simonneaux.
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