Seasonality in severity of depression: relationships to suicide and homicide occurrence.

Some previous studies have reported seasonal or monthly variations in the occurrence of depressive syndromes. The present study was carried out in order to investigate seasonality in severity of depression. Toward this end, the authors measured the Zung Self-Rating Depression (ZD) and Anxiety (ZA) Scales scores in 104 consecutively admitted depressed patients between November 1983 and April 1985. The data were analyzed by means of spectral analysis of a single time series. Up to 47.9% of the variance in the weekly average of the ZD scores could be explained by two significant rhythms of 51 (circannual) and 7 weeks. Peaks in ZD scores were observed in April-May, with lows occurring in August-September. Up to 30.8% of the variance in the weekly average of ZA scores was explained by a circannual rhythm. Our results show that there is a true seasonality in the severity of illness of depressed subjects. There were significant correlations between the weekly average in severity of illness and the chronograms of suicide (positively) and homicide (negatively) occurrence in Belgium.