The sample size in this study is very small, so results, though promising, can only be very preliminary.
On all parameters, the effects of exposure to low-intensity blue-enriched white light in the treatment of SAD did not differ from the effects of exposure to standard bright-light treatment. In this study, participants experienced blue-enriched white light as less comfortable than the standard light condition. In a previous SAD treatment study using blue-enriched white light of higher intensities, no differences in appreciation emerged between BLT and SLT [15
However, in this study, participants indicated they found the lower intensity blue-enriched white light slightly less pleasant than the standard light condition, a finding based on their answers on the evaluation questionnaire. In the SLT group 63% (7 out of 11) of the subjects rated the treatment as "rather pleasant" or "very pleasant". In the BLT group, treatment appreciation was slightly lower: 81% (9 out of 11) of the subjects in the BLT group indicated the treatment to be "neither pleasant, nor unpleasant", "rather pleasant" or "pleasant".
This small difference in appreciation in the present study is significant but must be interpreted within the context of the relatively modest sample size.
Moreover, in office settings, illuminances of around 360 lux, blue-enriched white light (17 000 °K) have been reported to improve subjective measures of irritability and eye discomfort as compared to white light of 4000 °K [29
Exposure to low-intensity blue-enriched white light (750 lux, 17 000 °K) is equally effective as standard full-spectrum light treatment (10 000 lux, 5000 °K). It would also have been interesting to make a direct comparison between the effects of exposure to standard light at 750 lux and at 10 000 lux. However, data of other studies indicate that higher-intensity light treatment leads to larger improvements than light treatment with lower-intensity light [2
Results are in line with those of Anderson et al. [16
], who found that blue monochromatic low-intensity light (98 lux) was equally effective in treating SAD as broadband white light at 711 lux with identical photon density in the 424-532 nm range. The short wavelength photon flux of these two conditions is highly comparable to the BLT condition of the present study. The current findings on exposure to low-intensity blue-enriched white light are also in line with the results of studies in office surroundings. It has been shown that sleep quality and alertness improved when workers spent their day in an office with blue-enriched white light with an intensity of 310.35 lux in the daytime instead of in the standard white (4000 °K) office lighting conditions with a mean intensity of 421.07 lux [29
]. In a similar study, the standard room lighting had an intensity of 345 lux and was compared to blue-enriched light with an intensity of 354 lux [31
As the biological clock is known to be highly sensitive to blue light, the phase-shift hypothesis suggests that blue-enriched light is a more powerful treatment for SAD than standard light. Although exposure to low-intensity blue-enriched white light in SAD patients leads to the same therapeutic results as exposure to the standard bright-light treatment, this does not necessarily indicates that blue or blue-enriched light has a more powerful influence on the biological clock. Studies by Smith et al. [32
], have demonstrated that, in a similar way, frequently used bright-light therapy photon densities (4.2 vs. 4.9 10E15 photons/cm2/s), blue-enriched white light (17 000 °K) does not outperform standard white light (4100 °K) in phase-advancing or phase-delaying effects. Interestingly, a recent study indicated that for irradiances between 2E12 and 1.5E14 photons/cm2/s, blue (460 nm) light does in fact outperform green light (555 nm) in phase-shifting effects, whereas blue light yields smaller phase-shifts than green light of identical photon density at lower intensities (in the 2.5E11-2E12 photons/cm2/s range) [34
An alternative explanation for our finding that the effects of blue-enriched light are not better than those of standard light is the possibility that the blue wavelengths are not necessary for the therapeutic effects of the treatment of SAD. Blue light plays a role in the working mechanism of the biological clock, but the role of the biological clock itself in the aetiology of SAD has not been fully established yet [35
Since this study has no placebo condition included, the similar responses to the two treatments could be interpreted as placebo effects only. It is impossible to create a real placebo condition for visible light treatment, though. The few studies testing light therapy in winter depressives using some kind of placebo condition (for example a deactivated negative-ion generator) revealed placebo effects that ranged from 21% to 41% [38
]. In a placebo-controlled study of extra-ocular light treatment, we found a placebo response of 36% [41
]. In this latter study, participants visited the clinic in the mornings for treatment, which was similar to the visits in this study. The response rates in the current study between 65% and 76% for remission are relatively high compared to the placebo responses from the placebo-controlled studies, probably too high to be interpreted as placebo effects only, although we can not rule out this possibility.