Latitude effect on bipolar temperaments

Abstract

Growing evidence points to an association of daily light exposure and both mood and mood disorders. In recent studies, we demonstrated that higher illuminance of daytime may be positively associated with hyperthymic temperament while lower illuminance of daytime may be positively associated with cyclothymic temperament. However, it is not possible to determine whether hyperthymic or cyclothymic temperament induces higher or lower illuminance via heliotropism or non-heliotropism or whether higher or lower illuminance induces hyperthymic or cyclothymic temperament via light effects. In order to elucidate these questions, we sought to compare bipolar temperaments between residents who live in regions with varying levels of sunshine. Japan has 4 large islands -Hokkaido, Honshu, Shikoku, and Kyushu. Sapporo in Hokkaido and Oita in Kyushu located at 43° and 33° of latitude differ significantly in mean yearly sunshine. We investigated the data of 189 subjects (94 from Sapporo and 95 from Oita) including their data of Temperament Evaluation of Memphis, Pisa, Paris and San Diego-auto questionnaire version (TEMPS-A). Multiple regression analyses revealed that latitude (Sapporo or Oita) effect predicted significant variance of hyperthymic temperament, but not of the other temperaments. The limitation of the present study was that there was a small but significant difference in age between the subjects of Sapporo and those of Oita. Therefore, we included age in the multiple regression analysis. Second, there were several climate factors other than sunshine which were significantly different between Sapporo and Oita. We cannot exclude these effects completely. Finally, we did not consider other factors such as psychosocial factors which might have affected bipolar temperaments. The present findings suggest that relatively lower latitude (potentially via more sunshine) may induce hyperthymic temperament.