Diurnal and seasonal variations in human platelet serotonin in man

Platelet serotonin (5-HT) was found to be an individually constant parameter correlating with the uptake of exogenous 5-HT. A pronounced diurnal pattern of 5-HT, with a minimum in the afternoon and higher values in the morning and evening, was similar to the diurnal variations in plasma free tryptophan. Platelet 5-HT was significantly higher at 8 a.m. than 4 p.m. in spring but no longer in autumn, and was similar in both men and women. It appeared that peripheral diurnal variations in 5-HT were opposite in phase to the central 5-HT rhythm.