Long, dark, cozy nights?

The human female is not a seasonal breeder. However, some seasonality in birthrates is an acknowledged phenomenon. But good research into a seasonal effect at time of conception is lacking, albeit not absent. For example, Cummings (2010) showed that exposure to light plays a role in the generation of lust in primates such as chimpanzees and baboons. A Swedish study (Carlsen et al., 2004) showed that men ejaculate more often when winter is over.

Unfortunately, we know little about the hormonal reasons for these observations. One could argue that possibly behavioural, rather than endocrinological, changes dominate when it comes to sexual urges. Spring and summer are simply accompanied by a better mood and a general joie de vivre.

Well, perhaps we need to rethink our, apparently, obvious assumptions in light of the study by Wesselink et al., appearing in the current issue of Human Reproduction. This study clearly indicates that dreary late fall, with its long, dark, cozy nights, rather than spring and summer, is the season where conception is more likely to occur.

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