Fig. 1: A 3-hit model of BD involving an interaction of risk genes (Hit 1) and life stressors (Hit 2) that is filtered through seasonal changes in photoperiod (Hit 3), represented as rates of change in solar insolation.

An idealized pattern of rates of seasonal changes in photoperiod for a location north of the equator is presented, and details relating to this measure may be found in Rosenthal et al. [42]. This model is a modification of the 3-hit model of vulnerability and resilience as originally proposed by Daskalakis and colleagues [62] and is based in part on the social zeitgeber theory of affective disorders [63,64,].