Seasonality in biochemical determinations: a source of variance and a clue to the temporal incidence of affective illness.

Systematic investigation of seasonal variations revealed unimodal annual rhythms in platelet serotonin uptake and DBH activity, bimodal seasonal rhythms in free and total tryptophan, melatonin, and platelet serotonin, and significant fluctuations of higher frequency in platelet MAO activity and protein. The methodological importance of seasonality is emphasized by the fact that the seasonal changes noted were often greater than differences previously found in the same parameters between depressive patients and controls. Since seasonal rhythms are circadian in origin, impairment of a central 24-hour biological clock could provide a common basis for circadian rhythm disturbances and seasonality in affective disorders.