Performing a placebo-controlled trial on the effects of BLT is challenging, since the appearance of the device or the characteristics of the emitted light can provide the subject with clues about the treatment condition. This can influence the subject’s expectancies of the treatment, which has been shown to impact on the outcomes of a study on BLT the past [
62]. Deactivated negative ionizers, dim red light and light boxes with full-band filters were used as control light conditions for BLT in previous studies [
37,
65]. In this trial, we also use a light device with full-band filter. However, the reduced intensity of the light could point out that the subjects in the control condition were not receiving BLT. Therefore, subjects’ expectancies are rated prior to the start of treatment, and participants are not informed about the exact details of the two light devices and their expected therapeutic effect. Although this might raise ethical questions, it is a valid way to guarantee subject blinding when studying a treatment that lacks a credible placebo condition. Therefore, this study did receive approval of the ethics committee. Moreover, the subjects in the control condition do receive some form of treatment. All subjects in our study are required to use the light device at fixed times in the morning and the evening, imposing a fixed sleep-wake structure on all participants. According to the ‘social zeitgeber’ hypothesis of Ehlers et al. (1988), a disruption of social rhythms can induce a depressive episode through disturbance of the circadian rhythm [
66]. Restoring the circadian rhythm by re-establishing daily routines, including time of awakening and going to bed, might therefore have a positive influence on mood [
66]. Since all participating subjects in our study achieve a more regular sleep-wake cycle, we feel that subjects in the control condition are not completely deprived of treatment. Moreover, a report by Zeiter et al. (2000) suggests that dim light of approximately 100 lux also has the ability to cause a shift in circadian rhythm [
67]. Although the healthy volunteers participating in this study were exposed to the light for 6.5 h [
67], we cannot rule out the possibility that 30 min of exposure to 200 lux of light will influence the circadian system of PD patients.
With our study design, we do not compare BLT to ‘pure’ placebo treatment, i.e. an ineffective control treatment. Rather, we compare structuring of the sleep-wake cycle through set treatment times in both conditions, with additional BLT in the experimental condition, and additional dim light in the control condition. In order to be able to assess the effects of BLT only, a future study on the effects of BLT on depression in PD would have two additional treatment arms, where BLT and control light are administered at random times. Moreover, more research is needed on the timing (morning or evening) and duration of BLT, as well as the influence of light intensity. However, we hypothesize that with the design of this study, we will still find a greater improvement on depression, insomnia and biomarkers of circadian rhythmicity in the subjects in our experimental condition as compared to our control subjects, due to the addition of BLT to the imposed sleep-wake structure.