It sounds like something from the spurious correlations website, but there is an association between how noisy a man's sleeping environment is, and the chances of him being fertile. Moreover, the authors of a paper in Environmental Pollution think the association is real, and a contributor to rising rates of male infertility.

Living in a noisy environment contributes to interrupted sleep, which in turn can have negative consequences from heart disease to loss of concentration. Nighttime noise exposure for pregnant women appears to contribute to premature birth and spontaneous abortion, and the same issue can make it difficult for women to get pregnant in the first place.

Male infertility is much less studied than its female equivalent, however, and that extends to the influence of noise. Kyoung-Bok Min of Seoul National University addressed this with a sample of more than 200,000 men aged 20-59 from a long-running Korean health study.

Among these men,1.6 percent were diagnosed as infertile. The raw data showed no relationship between noise levels and infertility over a four-year period. Once Min accounted for other relevant factors, however, such as smoking, blood sugar, age and particulate pollution exposure there was a statistically significant increase in infertility for men who experienced noise above 55 decibels (similar to light traffic) at night.

The association Min found was non-linear – small increases in sound levels below the 55 dB threshold did not appear to have any effect on sperm production or quality, perhaps because people get used to noises up to a certain level.

An association was also seen between infertility and exposure to slightly louder noises (60 dB) during the daytime, but this may reflect the fact that neighborhoods that are noisy at night are also likely to have more sound during the day.

"Infertility is becoming a significant public health issue because of unexpected adverse effects on the health and quality of life and heavy expenditures on the health system," said co-author Dr. Jin-Young Min. "We know noise exposure has an effect on male fertility in animals, but our study is the first to show the risk of exposure to environmental noise on male infertility in humans."

Many studies have reported declines, sometimes quite drastic ones, in human sperm counts from around the world. Other research has challenged these claims, but even among those scientists who accept that male infertility is rising, there has been extensive debate about the causes.

The work of the two Min's does not suggest that noise is the only, or even primary, factor making it harder for couples to get pregnant. However, it does indicate that for couples trying to conceive, sex is not the only bedroom activity that matters.