The results indicate a significant anxiolytic effect of acute bright light exposure. Moreover, the effect was consistent for each of the three days. The anxiolytic effect did not vary significantly by age group or sex of the participants. The corresponding effect size is considered small [27
], but comparable to other anxiolytic stimuli in low anxious adults [28
The data are consistent with other research suggesting a reduction of anxiety symptoms following bright light treatment of winter depression [20
]. A unique aspect of the present study is that the anxiolytic effects were observed after acute exposure to bright light, whereas others reports had considered effects after 2 or more weeks of light treatment.
The results are not consistent with a recent study by Goel and Etwaroo [30
], who found no significant effect of acute bright light treatment on the POMS-tension subscale, but a significant antidepressant effect was observed. The discrepancy in findings might be attributed to methodological differences. Whereas the present study involved 3 hr of exposure to 3,000 lux light and assessment of anxiety change at 20 min post-exposure, the Goel and Etwaroo study examined anxiety changes during exposure to 30 min of 10,000 lux light [30
There were several noteworthy limitations of the present study. First, an obvious methodological limitation was the lack of a control group, which was not possible in this ancillary open trial. Nonetheless, we do not think that the observed changes in STAI can be readily attributed to behavioral artifacts, such as demand characteristics or expectancy effects. Anecdotally, it seemed that the subjects viewed completion of the STAI as a small part of a complex study. As much focus was placed on following the ultra-short sleep-wake schedule, collection of urine samples, etc., it seemed that subjects were unaware of the research hypothesis of this ancillary study. Were demand or expectancy effects operating, greater anxiolytic effects might be expected. The STAI changes are likely also not attributable to experimentally imposed "time-out" from daily stressors, as the bright light treatment was not administered until the participants were in the laboratory for = 30 hr. Nonetheless, controlled, randomized studies are needed to verify these findings.
A second limitation was the low baseline levels of anxiety of the participants. As expected, the few high-anxious participants had far greater anxiolytic responses than the other participants. Despite receiving recommended standardized instructions for completing the STAI, a remarkably high number of participants were excluded from the analyses due to low baseline STAI levels (< 25). The decision to exclude these individuals was made a priori. The reasons for the low baseline STAI levels are unclear. The participants might have made efforts to complete the STAI questionnaires in a socially desirable manner. However, similarly abnormally low values were not observed for the CES-D data. The lack of inclusion of individuals ages 31–58 yrs, who tend to have high rates of anxiety, might have contributed to low baseline levels.
A third limitation was that the high levels of health and fitness of the participants was not representative of the population, particularly not that of older individuals. However, this limitation might have led to an underestimation of the efficacy of bright light, which have been more clearly demonstrated in less healthy individuals. Some correlates of health and fitness, such as better psychological health and greater habitual exposure to bright outdoor light, might have attenuated the anxiolytic effects of bright light.
A fourth limitation might have been the unique laboratory environment for exploring psychological benefits of bright light, involving prolonged maintenance of the ultra-short sleep-wake cycle, a moderate degree of sleep deprivation, low levels of light exposure, etc. However, despite the rigors of the experiment, it did not appear to be "stressful". For example there were no changes in pre-treatment STAI levels between DAYs 1–3. Indeed, many subjects reported that the experience was calming. As addressed above, by separating the subjects from their usual daily stressors, the environment might have resulted in an underestimation of the anxiolytic effects of bright light treatment.
That participants had generally had low levels of depression suggests that the anxiolytic effect of light exposure was independent of significant antidepressant effects. Moreover, in post-hoc analysis, we found no significant correlation of anxiolytic effect with baseline level of depression (r = 0.043).