Annual Rhythm of Human Reproduction: II. Environmental Correlations

Abstract

Annual rhythms of human conception rates (based on a worldwide selection of statistics) were correlated with photoperiod, monthly averages of daily hours of sunshine, minimum and maximum temperature, and humidity. Our results show for the first time on a global scale that photoperiod, as shown for many animals, may also influence the physiology of human reproduction. At higher latitudes, where changes in daylength are pronounced, a steep increase in human conceptions coincides with the vernal equinox. Temperature also appears to be a major influencing factor. Conception rates are above the annual mean at temperatures between 5 degrees and 20 degrees C, and temperature extremes decrease the probability of conceptions. In regions with cold winters and moderate summers, the number of conceptions correlates positively with temperature; close to the equator, where winters are more moderate than the hot summers, this correlation is negative. Regions with both hot summers and cold winters tend to have a bimodal conception rhythm. The influence of photoperiod is dominant before 1930, whereas the impact of temperature dominates in later years. With industrialization, people are increasingly shielded from both photoperiod (by indoor work) and temperature (by heating and air conditioning), which may explain the deseasonalization of the human conception rhythm. Photoperiod and temperature and the specific changes in their impact can account for several features of the annual human conception rhythm: latitude dependence, waveform, phase and amplitude, and their specific changes over time.

Demasking biological oscillators: Properties and principles of entrainment exemplified by the Neurospora circadian clock

“…A two-component cosine function was fitted to each profile. Composite cosine fits using the fundamental and the first harmonic are extremely powerful in describing most waveforms (17). The average correlation coefficient of these least-square fits was r ϭ 0.93 Ϯ 0.07.…”

Clock genes in calendar cells as the basis of annual timekeeping in mammals--a unifying hypothesis

“…Humans express all elements of photoperiodism and circannual rhythm generation, albeit in a relatively weak fashion compared with many non-primate species (Roenneberg & Aschoff 1990, Wehr 2001. In man, the cyclicity is revealed as seasonal cycles in birth season, incidence of twinning, semen quality, carbohydrate metabolism and weight gain, and notably as behavioural traits of clinical importance including seasonal affective disorder and bulimia nervosa (Wehr & Rosenthal 1989, Blouin et al 1992, Wirz-Justice et al 2001.…”

Daily, monthly and seasonal variation in PSA levels and the association with weather parameters

“…Sexual behaviour in humans does show seasonal variation. [13][14][15][16][17] This is apparent in many European countries, 13 with peak conception rates occurring in April/May. Conception rates also correlate with increasing exposure to sunlight and with temperature.…”

“…Conception rates also correlate with increasing exposure to sunlight and with temperature. 14,15 Further, the frequency of sexual activity, rate of sexually transmitted diseases and the sale of contraceptives are all seasonal, 16 with Carlsen et al 17 demonstrating higher rates of ejaculation during springtime in Danish men. Therefore, seasonal variation in PSA levels may be a reflection of the variation in ejaculation frequency in the studied populations.…”

“…Therefore, seasonal variation in PSA levels may be a reflection of the variation in ejaculation frequency in the studied populations. It is interesting that, Roenneberg and Aschoff found that conception rates are decreased at extremes of temperature, 14 which, if there was an associated decrease in frequency of ejaculation, may explain the lower levels of PSA in Spain during the summer and the subsequent higher levels in autumn and winter. 8 A similar argument can be made for physical activity to be a contributing factor to seasonal variation in PSA.…”