Hypothetical model connecting photoperiodic input with circadian (SCN), mood (DRN), and reward (VTA, NAc) systems and the associated psychiatric disorders. A) We hypothesize that that the SCN acts by mediating circadian entrainment under Long summer-like photoperiods (Ciarleglio et al., 2011b) and impacting the DRN, VTA, and NAc resulting in a potential decreased risk for anxiety, depression, or substance use disorders. B) We hypothesize that that the SCN acts by mediating circadian entrainment under Short winter-like photoperiods (Ciarleglio et al., 2011b) and impacting the DRN, VTA, and NAc resulting in a potential increased risk for anxiety, depression, or substance use disorders.