Background: Seasonal affective disorder (SAD) is characterized by recurrent episodes of major depression with a seasonal pattern, treated with light therapy (LT). Duration of light therapy differs. This study investigates retrospectively whether a single week of LT is as effective as two weeks, whether males and females respond differently, and whether there is an effect of expectations as assessed before treatment. Methods: 83 women, and 25 men received either one-week (n¼42) or two weeks (n¼66) of LT were included in three studies. Before LT, patients’ expectations on therapy response were assessed. Results: Depression severity was similar in both groups before treatment (F(1,106)¼0.19 ns) and
decreased significantly during treatment (main effect “time” F(2,105)¼176.7, po0.001). The speed of therapy response differs significantly in treatment duration, in favor of 1 week (F(2,105)¼3.2, p¼0.046). A significant positive correlation between expectations and therapy response was found in women (ρ¼0.243, p¼0.027) and not in men (ρ¼ 0.154, ns). When expectation was added as a covariate in the repeated-measures analysis it shows a positive effect of the level of expectation on the speed of therapy response (F(2,104)¼4.1, p¼0.018). Limitations: A limitation is the retrospective design. Conclusions: There is no difference between 1 and 2 weeks of LT in overall therapy outcome, but the speed of therapy response differed between 1 week LT and 2 weeks LT. Together with the significant correlation between expectations and therapy response in women, we hypothesize that expectations play a role in the speed of therapy response.