There is evidence for the reciprocal interaction between circadian oscillation and reproduction, and disruption of circadian rhythms has been associated with impaired menstrual functions and reduced fertility in women. However, only little information is available on the relationship between reproduction and chronotype. The aim of the present study is to better assess this relationship. The participants (aged 25 to 74 yrs) were selected randomly from the Finnish Population Information System. The data from 2672 female participants of the National FINRISK Survey 2007 were analyzed to test the associations between chronotype (morning, intermediate, or evening) and reproductive features. Of the participants, 139 (5.6%) were evening, 1217 (48.7%) intermediate, and 1145 (45.8%) morning chronotypes. Among the participants aged 25 to 54 yrs, the duration of menstrual cycle was longer among evening chronotypes (28.8 ± 4.4 d) than among morning (27.7 ± 2.6 d; p < 0.01) and intermediate (27.8 ± 3.3 d; p = 0.05) chronotypes. Significant correlations were found between the higher morningness-eveningness scores (the more of morning chronotype) and the shorter durations of menstrual bleeding, both in the whole sample (p < 0.001) and after limiting the analyses to women younger than 55 yrs (p < 0.05). In multivariable analyses on the whole sample, as compared with morning chronotypes, intermediate chronotypes had a significantly longer duration of menstrual bleeding (B = 0.160, 95% confidence interval [CI] = 0.044 to 0.276; p < 0.01) as well as a higher odds for difficulties in getting pregnant (odds ratio [OR] = 1.464, 95% CI = 1.118 to 1.917; p < 0.01). Our findings suggest that chronotype is related to the reproductive function in women.
