The palliative effects of exercise on depression are neither generally accepted nor clearly understood. On the contrary, the efficacy of light exposure in the treatment of seasonal depressive symptoms is widely recognized. It is possible that the contradictory findings in the exercise and depression literatures are due to a failure to consider light exposure as a variable. 32 women who regularly exercised were administered a depression inventory and the Seasonal Light Inventory, a new measure composed of two 10-item subscales, Seasonality and Light Exposure. Seasonality assesses fluctuations of mood, appetite, sleep, and other activities occurring at similar times each year, and the Light Exposure subscale measures respondents' estimated exposure to outdoor and bright indoor light. Indeed, those exercisers whose habits exposed them to a relatively large amount of light reported fewer seasonal depressive symptoms than those whose habits exposed them to a relatively small amount of light. The results suggest that the puzzling relationship between exercise and depression might be delineated more clearly by considering light exposure as a confounding variable.