Dose-dependent effects of light on hyperthymic temperament

In a previous study examining the association of hyperthymic temperament and daily light exposure (illuminance), we compared hyperthymic scores of the Temperament Evaluation of Memphis, Pisa, Paris and San Diego-auto questionnaire version (TEMPS-A) acquired from residents in Sapporo and Oita in Japan, which are located at 43° and 33° of latitude. We found that residents of Oita had significantly higher hyperthymic scores than residents of Sapporo. Moreover, a comparison of 3 regions rather than 2 regions may show a dose-response relationship in hyperthymic temperament and illuminance.

Japan has 4 large islands-Hokkaido, Honshu, Shikoku, and Kyushu. Sapporo is in Hokkaido and Oita is in Kyushu. As the 3rd location, we recently collected TEMPS-A data from 125 residents in Koshigaya which is located in Honshu at 36°. We combined the data of the Koshigaya residents with the data of 94 residents from Sapporo and 95 from Oita.

The mean yearly total sunshine of the past 20 years (1993-2012) was 1684.6h in Sapporo, 1862.9h in Koshigaya, and 2002.9h in Oita. Multiple regression analyses revealed that sunshine predicted significant variance of hyperthymic temperament in a dose-dependent manner.

The limitation of the present study is the lack of the consideration of the effects of temperature on hyperthymic temperament. Since correlations are fairly small, other factors are also at work for hyperthymic temperament.

The present findings suggest that higher illuminance may maintain hyperthymic temperament via light effects in a dose-dependent manner.