Have you ever wondered why you have such a hard time falling asleep at night and getting up in the morning? Why do babies take so long to learn how to sleep through the night? How does jetlag affect your entire body? Is there an optimal time to eat? To exercise? To take medicine? Why are some people better at working the night shift (or pulling an all-nighter) than others? At the root of all of these questions is a biological timekeeping system, responsible for synchronizing and regulating the timing of physiology such as metabolism, sleep, hunger, and reproductive cycles.

Biological timing systems are vital for successful reproduction. Animals are more likely to gain mating opportunities if they coordinate their sexual receptivity with that of their potential partners. Females benefit from synchronizing the timing of pregnancy to seasons with favorable food and weather conditions, and it is advantageous for an animal to give birth at a time of day when it is most likely to be in inactive and resting.

Human female fertility and reproduction is also regulated by the internal timing system. Disruptions of the daily 24-hour (circadian) clock, which often occurs in shiftworkers, can cause delays in conception and increases in miscarriages and premature births when compared to non-shiftworkers. Breast cancer survivorship is improved dramatically when medical treatment is timed to particular stages of the menstrual cycle. The precise timing of these reproductive events is regulated by a small number of cells also located in the hypothalamus, in a region called the suprachiasmatic nucleus (SCN). This cell group acts as the body's master clock and coordinates the timing of the hormonal and neuroendocrine surge as well as a large number of other biological rhythms.

In a second line of research we examine how exposure to steroid hormones in utero can alter the expression of proteins or genes in male and female animals. Environment steroids are found in pesticides, industrial chemicals, and naturally occurring plant hormones, thus wildlife, domestic species, and humans regularly come in contact with these hormones or hormone mimics. These chemicals induce profound defects in reproductive processes in both adults and fetuses, even when individuals are exposed to relatively low doses.

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