Contribution of the photoperiod at birth to the association between season of birth and diurnal preference.

The aim of the present study was to evaluate the influence of season of birth and photoperiod at birth on diurnal preference in young adults. Participants were 1591 volunteers aged 17-35 years (69% women). The scores obtained on the Morningness-Eveningness Questionnaire (MEQ) of Horne and Ostberg were first assessed as a function of gender and season of birth by a two-way ANOVA. Results revealed a higher MEQ score (reflecting more morningness) in women than in men, and in individuals born during autumn than in those born during spring. There was no gender-by-season interaction. The MEQ scores of four categories of photoperiod at birth were also compared. Individuals born during short photoperiods of 8-10h showed stronger morningness than individuals born during photoperiods of 12-14 h. The highest morningness scores were observed in subjects born during 8-10h photoperiods in the autumn. By contrast, diurnal preference of subjects born during 8-10h photoperiods in the winter did not differ from diurnal preference of subjects born during longer photoperiods. Our results add support to previous reports showing an association between season of birth and morningness-eveningness, but this association cannot be explained entirely by the length of the photoperiod at birth. We suggest that the association between season of birth and diurnal preference reflects an influence of light intensity and/or variations in photoperiod during early development on the characteristics of the circadian system.