Seasonal pattern of melatonin excretion in humans: relationship to daylength variation rate and geomagnetic field fluctuations

In order to investigate the influence of various environmental parameters on melatonin excretion, the night-time urinary melatonin excretion of 16 healthy volunteers was measured in samples collected monthly over a period of one year. No significant interindividual differences were detected in the monthly rate of change of melatonin excretion. A seasonal bimodal pattern did, however, emerge. Peak values were observed in June and November. In these months a combination of high daylength stability and low values of the vertical component of the geomagnetic field was recorded. Trough values were found in April and August–October when low daylength stability was combined with high values of the vertical component of the geomagnetic field. We propose that the daylength variation rate, and the fluctuations of the vertical component of the geomagnetic field, interact to induce the changes in melatonin secretion which signalize the different seasons in humans.