Bright light treatment: are we keeping our subjects in the dark?

Bright light treatments for seasonal affective disorder (SAD) and phase-disturbed circadian rhythms show considerable interindividual variation in response. Many studies have explained this variation in terms of traditional explanations of response variance, i.e., poor diagnostic criteria, poor compliance, etc. This study examined proximity and gaze behavior during illumination as a source of variance in response. Distance from the light source and gaze were varied systematically in order to quantify the discrepancy between perceived and transmitted light. The results indicate that the discrepancy between perceived and transmitted light increases rapidly as a function of distance and gaze. At best, subjects exposed to a transmitted illuminance of 10,000 lux perceived only 20% of the transmitted light, and this dropped rapidly to between 1 and 2% when subjects were more than 3 feet away and not gazing directly at the light source. These results suggest that light treatment protocols may be compromised by lack of attention to compliance and administration factors. These issues may account for much of the response variance observed previously.