The significance of winter depression and summer depression in the evolution of bipolar disorders

Seasonal rhythms are an understudied area of Psychiatry. We review winter depression and summer depression (formerly called seasonal affective disorders) with an eye toward understanding why they arose and how they are related to each other. These two entities, particularly in their subsyndromal forms, may hold great evolutionary significance. We will show how winter subsyndromal depression, found primarily in temperate zones, may have served to foster healthier pregnancies, while summer subsyndromal depression, found primarily in tropical zones, likely served to prevent pregnancies. The hypomanias associated with each—with their activation and intensified sexual drive—were a means to skew conceptions to optimal times. Seasonal rhythms, as epitomized by winter depression and summer depression, have enabled our colonization of the planet and—it will be argued—may have given Homo sapiens the very mixed blessing of bipolarity.