Shift work and pregnancy: night light, baby not right

Field: Medicine (Health Sciences)

Pregnancy is a complex but successful biological strategy that ensures survival of mammals. Such a heavy burden is accomplished through a remarkable myriad of functional maternal adaptations involving the whole organism. It is well established that adverse situations during pregnancy, besides compromising maternal health, are associated with long-term effects on the offspring's health. The effects of malnutrition (excess and deficits) during pregnancy are thought to contribute to current world-wide public health problems, encased in the concept of metabolic syndrome such as obesity and hypertension (reviewed in Hanson & Gluckman, 2014). Of late, the potential negative effects of maternal exposure to the disruption of the normal day-night cycle in which we earthlings evolved, as in shift work, is gaining much attention. Epidemiological studies associate shift work with increased risks of chronic disease including diabetes and obesity (Proper et al. 2016). Shift work interferes with normal patterns of sleep-activity, feeding-fasting and light-dark exposure. It has been proposed that the exposure to light at night leads to desynchronization of the multi-tissue molecular circadian clock machinery controlling circadian biological rhythms. Thus, the tidy progression of overt circadian rhythms like temperature, sleep, plasma melatonin/ACTH/corticosteroid secretion, metabolic status and feeding through the 24 h tuned to the environmental daily light-dark cycle is altered, leading to problems ranging from the unpleasant effects of jet lag to the more severe effects already mentioned. Experimental studies in rodents show that simulated shift work increases adiposity and perturbs glucose metabolism. Moreover, there is compelling evidence that manipulation of the photoperiod to which pregnant dams are exposed affects the mother and the fetus and leads to abnormal offspring physiology, including circadian disruption (Varcoe et al. 2013, 2018, Mendez et al. 2016, Salazar et al. 2018). However, caution is advised in extrapolating these important findings to humans and other species. From a teleological point of view, fail-safe mechanisms may be operating in animals such as sheep, humans, elephants, whales, etc. carrying one or two fetuses during a long pregnancy in which many negative events may randomly occur to the mother. Given the fact that women make up a large portion of the workforce, the exploration of the consequences of shift work during pregnancy is imperative. As reported in this issue of The Journal of Physiology, Gatford et al. 2019 undertook this challenge by studying maternal effects of chronic photoperiod shifts during pregnancy in the ewe. Sheep have a long pregnancy (about 20 weeks), are diurnal and carry one or two fetuses. Additionally, the benchmarks of lamb fetal development are similar to the human. The authors developed an experimental protocol of simulated shift work (SSW) imposed on the first third, first and second third and during the entire pregnancy, mimicking exposure to working conditions for pregnant women who may be subjected to shift work (nurses, physicians, flight attendants, police, pilots and even circadian researchers). Results in early pregnancy are important as most women will continue their jobs during early gestation. The study provides very important and novel longitudinal information on aspects of circadian/diurnal maternal physiology in control ewes throughout gestation. Diurnal rhythms of activity, plasma melatonin, glucose and insulin are present at weeks 7, 14 and 19 of pregnancy. Moreover, diurnal rhythmic changes in clock gene expression are present in muscle, an accessible peripheral circadian clock. These in vivo findings outline the presence of a normal circadian system throughout pregnancy under control light-dark conditions in the ewe. The studied rhythms do not change with advancing gestation. Of note, the amplitude of the circadian melatonin rhythm does not increase with advancing gestation as has been reported to occur in rodents and humans. Given that data in the literature is limited, it suggests that this issue should be explored further. The study clearly demonstrated that SSW alters diurnal rhythms of the variables studied. Differences in recovery time to achieve a normal pattern between activity, melatonin and clock gene expression in muscle were noted, suggesting that SSW disrupted internal synchrony between central and other possible peripheral clocks, besides muscle. Additionally, how these disturbances affected maternal metabolic physiology (tested as the response to a glucose challenge test), pregnancy length and newborn weight was also studied. An important and worrisome finding was that SSW during the first third of gestation altered the response to a glucose challenge test in ewes carrying singleton pregnancies and lowered the lambs birth weight. These findings raise concern, as crucial events in fetal development take place in early gestation and most pregnant women work during this stage of gestation. Interestingly, there is capacity for adaptation of maternal metabolic function to SSW as the glucose challenge test normalized at late gestation, despite the altered circadian function. Up to now, the information available in rats, and in sheep, as reported by Gatford et al. 2019 in this issue, points to important effects of chronic phase shifts of the photoperiod on maternal physiology. As mentioned previously, long-term effects in the offspring of rodents have been shown. We will certainly see the follow-up to the present study in sheep, in newborns and beyond. No competing interests declared. Sole author. Project 16-2017, Valley Obstetrics and Gynecology Medical Group Inc. Colton, CA, USA. I thank Alexandra and Joseph Mele and Monica Prizant for title suggestions.