Disturbed circadian rhythms have been observed in seasonal affective disorder (SAD). The aim of this study was to further investigate this connection, and to test for potential association between polymorphisms in circadian clock-related genes and SAD, seasonality (seasonal variations in mood and behavior), or diurnal preference (morningness-eveningness tendencies). A total of 159 European SAD patients and 159 matched controls were included in the genetic analysis, and subsets were screened for seasonality (n ¼ 177) and diurnal preference (n ¼ 92). We found that diurnal preference was associated with both SAD and seasonality, supporting the hypothesis of a link between circadian rhythms and seasonal depression. The complete case-control material was genotyped for polymorphisms in the CLOCK, Period2, Period3, and NPAS2 genes. A significant difference between patients and controls was found for NPAS2 471 Leu/Ser (w 2 ¼ 9.90, Bonferroni corrected P ¼ 0.035), indicating a recessive effect of the leucine allele on disease susceptibility (w 2 ¼ 6.61, Bonferroni corrected P ¼ 0.050). Period3 647 Val/Gly was associated with self-reported morningness-eveningness scores (n ¼ 92, oneway ANOVA: F ¼ 4.99, Bonferroni corrected P ¼ 0.044), with higher scores found in individuals with at least one glycine allele (t ¼ 3.1, Bonferroni corrected P ¼ 0.013). A second, population-based sample of individuals selected for high (n ¼ 127) or low (n ¼ 98) degrees of seasonality, was also genotyped for NPAS2 471 Leu/Ser. There was no significant difference between these seasonality extreme groups, and none of the polymorphisms studied were associated with seasonality in the SAD case-control material (n ¼ 177). In conclusion, our results suggest involvement of circadian clock-related polymorphisms both in susceptibility to SAD and diurnal preference.

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These results do not suggest a major role of the short variant of 5-HTTLPR in susceptibility to SAD, but provide modest evidence for an effect on seasonality.