With around 12 percent of women of reproductive age having trouble conceiving, infertility is a serious problem in the United States and throughout the world. Many cases of infertility are idiopathic, which means doctors cannot identify a reason for the difficulty in conceiving. However, a new study published in the Journal of Occupational and Environmental Medicine suggests that working long hours and otherwise disrupting your natural rhythms may be a possible cause of infertility.

Infradian Rhythms and Fertility

All mammals have fertility cycles that are controlled by an infradian rhythm. An infradian rhythm is simply one that is longer than a circadian, or 24-hour, rhythm. Some animals have estrus, or heat cycles, while many primates have menstrual cycles. Human women are fertile in a cycle that roughly matches the cycles of the moon and the tides.

Most people are familiar with menstrual cycles but few realize how important these are to female fertility. In a healthy cycle, women release hormones sequentially in a manner that supports optimal fertility. If this cycle is disturbed, both health and fertility may suffer.

The Problem With Shift Work

Nurses have long been studied by the Nurse’s Health Study, a multiple decade study that has offered insight into a variety of health issues. This study also tracked fertility, among other outcomes. In this study, nurses who work the night shift took around 20 percent longer to become pregnant. This amount of time, while seemingly insignificant, represents many days of watching and waiting when extrapolated to all of the people studied.

While this study identified a correlation between shift work and lowered fertility, a new study was needed to pin the problems on working odd hours and not the multiple other stresses of nursing life.

Can Heavy Lifting Impact Fertility?

It is important to get a healthy amount of both aerobic and weight-lifting exercise. However, too much of a good thing is rarely good, and this area is no exception. People who perform more heavy lifting are less fertile. In fact, women who perform vigorous aerobic exercise are 42 percent less likely to get pregnant as those who do not exercise at all. One important note: This only applies to women with an optimal BMI and not women who are obese.

Researchers have several hypotheses as to why this is the case. In general, weight-bearing and aerobic exercise are good for health. However, exercising excessively may disrupt the infradian rhythm. The pituitary gland shuts down certain aspects of the menstrual cycle in response to physical stress. It’s possible that the body interprets heavy lifting as stress even when the exercise is planned and performed purposely. In addition, vigorous exercise can negatively affect the ability of a fertilized egg to implant and set down roots in the womb.

Other Factors in Shift Work and Fertility

These study findings, while promising, should be interpreted with reservation. There are so many factors in each woman’s daily life that can affect fertility. For example, women who drink caffeine are more likely to suffer fertility problems, and people who work odd shifts often use caffeine as a coping mechanism. In addition, people who are obese are more likely to suffer infertility, while obesity is more common in people who work odd shifts. This new study does not answer questions so much as presenting new and interesting ones.

Infradian rhythms are crucial to fertility. We are only just beginning to understand the intertwined nature of a woman’s biological rhythm. However, researchers in chronobiology agree that working outside of a natural circadian rhythm may negatively affect monthly fertility cycles and thus affect conception.