Circadian rhythms and fertility

► Circadian rhythms and clock gene expression are evident in hypothalamic/pituitary function. ► The ovary but apparently not the testis exhibit cellular rhythmicity. ► Disruption of clock gene function can reduce fertility. ► While the rhythmicity evident in rodents is not overt in humans, disruption to rhythms by shiftwork may impair fertility. Circadian rhythms impact on a wide range of physiological systems and this impact extends to fertility, such that disruptions to timing systems can impact upon reproductive capacity. This is highlighted most obviously in mutant mouse models whereby deletion or mutation of single genes results not only in disrupted circadian rhythmicity, but also compromised male and female reproductive function. In this review, we discuss the presence of circadian clocks in female and male reproductive tissues and the role these clocks play in the generation of oestrus cycles, ovulation, sperm generation, implantation and the maintenance of pregnancy. Given the increased incidence of shiftwork and international travel which disrupt circadian rhythmicity, and the increasing prevalence of reproductive technologies whereby early embryo development occurs without external time cues, it is important for us to consider the role of circadian rhythms in fertility.