ObjectiveTo investigate the effects of light therapy on serotonin transporter binding (5-HTT BPND), an index of 5-HTT levels, in the anterior cingulate and prefrontal cortices (ACC and PFC) of healthy individuals during the fall and winter. Twenty-five per cent of healthy individuals experience seasonal mood changes that affect functioning. 5-HTT BPND has been found to be higher across multiple brain regions in the fall and winter relative to spring and summer, and elevated 5-HTT BPND may lead to extracellular serotonin loss and low mood. We hypothesized that, during the fall and winter, light therapy would reduce 5-HTT BPND in the ACC and PFC, which sample brain regions involved in mood regulation. MethodIn a single-blind, placebo-controlled, counterbalanced, crossover design, [C-11]DASB positron emission tomography was used measure 5-HTT BPND following light therapy and placebo conditions during fall and winter. ResultsIn winter, light therapy significantly decreased 5-HTT BPND by 12% in the ACC relative to placebo (F-1,F-9=18.04, P=0.002). In the fall, no significant change in 5-HTT BPND was found in any region across conditions. ConclusionThese results identify, for the first time, a central biomarker associated with the intervention of light therapy in humans which may be applied to further develop this treatment for prevention of seasonal depression.