Background: Converging lines of evidence have established a relationship between exposure to ambient light and both mood state and mood disorders. In view of the association between mood disorder and affective disposition, in this study, the relationship between temperament (depressive, cyclothymic, hyperthymic, irritable and anxious) and daily light exposure was closely examined. Methods: Fifty-six subjects completed the Japanese standardized version of the Temperament Evaluation of Memphis, Pisa, Paris and San Diego-auto questionnaire version (TEMPS-A) and illuminance of daytime using actigraphy. Results: Cyclothymic temperament scores were significantly and negatively associated with illuminance of daytime whereas hyperthymic temperament scores were significantly and positively associated with illuminance of daytime. Limitations: Sample size was relatively small and they were only healthy subjects and no patients. Conclusions: The findings suggest that illuminance of daytime may be related to both cyclothymic and hyperthymic temperaments but that the associations lie in opposite directions. If this is the case, it seems important to consider individual patients' temperaments when light therapy is used to treat their depressive episode. Further studies are required to determine why and how these temperaments are associated with illuminance. (C) 2011 Elsevier B.V. All rights reserved.