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Rhythm characteristics in the about-daily (circadian) and about-yearly (circannual) frequency ranges were assessed for urinary melatonin. Clinically healthy women in Minnesota, USA, and Kyushu, Japan, were sampled around the clock once in 1-4 seasons. Possible differences that could reflect the large difference in breast cancer incidence in these two geographic locations were investigated. Each subject's risk of developing breast cancer, cardiovascular diseases resulting from an elevated blood pressure, and emotional conditions was numerically evaluated according to epidemiologic questionnaires. A prominent circadian rhythm characterizes urinary melatonin in both populations, peaking in the middle of the night. The American women exhibit a larger circadian rhythm-adjusted mean (mesor) than do the Japanese women. A circannual rhythm is also apparent in the North American women, but not in the Japanese women. The circadian mesor of urinary melatonin correlates negatively with the risk score of emotional depression and positively with that of developing cardiovascular diseases.

Urinary melatonin excretion is lower in East-Asian (Japanese) than in North-American (whites of mixed ethnic origin) women. Moreover, a statistically significant circadian rhythm is demonstrated by population-mean cosinor in the data pool from both groups of women. Furthermore, statistical significance characterizes interactions of effects from geographic differences (between ethnic groups) with temporal factors. Such spatio-temporal interactions await further scrutiny with a view inter alia of carcinogenesis as it is influenced by a spectrum of intermodulating rhythms.