Circadian rhythms of 6-sulphatoxy melatonin, cortisol and electrolyte excretion at the summer and winter solstices in normal men and women

Urinary excretion of 6-suphatoxy melatonin, cortisol, potassium and sodium was monitored at four hourly intervals for 24 h in 30 normal subjects at the summer and winter solstices. The 24 h profiles were fitted to sine curves and mean 24-h excretion, time of maximum excretion and amplitude of the curves compared. The excretion of 6-sulphatoxy melatonin was remarkably stable at the two times of the year (24-h excretion 108 ± 6.3 nmol in summer and 105 ± 6.3 nmol in winter, mean ± sem). The time of maximum excretion was significantly delayed in winter by 1 h and 40 min. Urinary cortisol excretion was significantly higher in winter, however, the amplitude was unaltered. The time of maximum excretion of cortisol was significantly delayed by 1 h and 34 min. Potassium and sodium excretion were both unaffected by seasonal influences. These results contrast with results in some animal species in which the duration of the melatonin signal is thought to be the key determinant in subsequent melatonin action. In humans it is likely that the phasing of the melatonin rhythm is of prime importance.