Re-analysis of the association of temperature or sunshine with hyperthymic temperament using lithium levels of drinking water

Abstract Background The Japanese archipelago stretches over 4000 km from north to south and has four large islands: Hokkaido, Honshu, Shikoku, and Kyushu. Previously, using the Temperament Evaluation of Memphis, Pisa, Paris and San Diego-auto questionnaire version (TEMPS-A), we compared the hyperthymic scores of residents in Sapporo, Obihiro, Takaoka, Koshigaya, and Oita cities (which are located at latitudes of 43°N, 42°N, 36°N, 36°N and 33°N with various combinations of ambient temperament and sunshine in Japan, respectively). We found that latitude predicted significant variance in hyperthymic temperament, and that ambient temperature, but not sunshine, significantly affected hyperthymic temperament scores. However, the analysis failed to consider the effects of naturally occurring low-dose lithium on temperament. Methods In addition to the TEMPS-A data previously collected, we measured lithium levels of the five cities. The effect of temperature, sunshine, and lithium levels on hyperthymic temperament was analyzed for the five cities. Results A stepwise multiple regression analysis revealed that lithium levels as well as latitude, but not temperature or sunshine, predicted significant variance in hyperthymic temperament scores. Hyperthymic temperament scores were significantly and positively associated with lithium levels whereas they were significantly and negatively associated with latitude. Limitations The light, temperature, lithium exposure that residents actually received was not measured. The number of regions studied was limited. The findings might not be generalized to residents across Japan or other countries. Conclusions The present findings suggest that lithium in drinking water may positively maintain hyperthymic temperament, and that latitude may negatively maintain it.