It's too darn bright.

Three centuries of birth records in the southern part of the Netherlands indicate that conceptions (and, one may infer, heterosexual intercourse) occur most often in the spring; but a study of nearly a million Pap smears taken between 1983 and 1998 shows that women in this region are about twice as likely to contract human papillo-mavirus infection in August as in winter. (1) Evidence o f infection fell off abruptly every September. Meteorological data confirmed that in each year of the study period, the amount of sunlight available peaked in August. The researcher speculates that the increased incidence of infection during the summer is linked to the immune-suppressing effects of ultraviolet light. He emphasizes that it is important to confirm this relationship, "because whatever risk is conferred by sunlight is, in principle, behaviorally avoidable."

(1.) American Association for Cancer Research, Increasing the risk of cancer: new research sheds surprising results on potential sources of cancer risk, news release, Philadelphia: American Association for Cancer Research, Mar. 30, 2004.