Annual menarche and scholar rhythms

Background: It has been hypothesized that the yearly menarche rhythm could be caused by the seasonal variation of photoperiod and temperature or by th e annual distribution of the scholar vacation and study periods. Aim: To te st the hypothesis that the distribution of study vacation periods is a cond ition that modifies the annual menarche rhythm. Subjects and methods: Two t housand ninety four school girls from Chile, 2.356 girls from Madras, India , 3.454 girls from Medellin, Colombia and 2.627 girls from Debrecen, Hungar y, were studied. They were asked about the month of their menarche. Vacatio n months were considered those with more than 6 days of leave from school. Results: the seasonal hypothesis was refuted because there were contradicti ons with the expected antithetical behavior in both hemispheres, there was a significant heterogeneity of the yearly menarche among girls from the sam e region, the expected cline of the menarche frequency variance from equato r to poles was not observed, finding an antithetical cline instead and ther e was a significant heterogeneity among months of the same season. On the o ther hand, months with vacation periods coincided significantly with peaks of menarche, while study months had lower proportion of menarche (total bin omial probability <10-6). Girls whose month of menarche was the same as the ir month of birth, did not agree completely with the vacation-study hypothe sis as the rest of the sample. Conclusions: Vacation periods influenced men arche rhythm. However these periods coincide with most cultural events and this strong association needs further study to be considered causal. It is not possible to assume school stress as the main explanatory variable. Onto genetic factors such as birth imprinting also can influence the menarche rh ythm. as shown in girls whose month of menarche coincided with their month of birth.