Large GWASs on chronotype and mood/sleep disorders have failed to directly implicate PER3 [28], but numerous candidate gene studies have found associations between polymorphisms in the PER3 gene and anxiety (e.g., [29]), depression (e.g., [10,23,30]), seasonal affective disorders (e.g., [15,31]), and sleep disorders (e.g., [15,16,32,33]), suggesting that there may be a threshold effect requiring multiple synergistic interactions between variants. Here, PER3 is linked to major depressive disorder.