Chromaticity of daylight: is the spectral composition of daylight an aetiological element in winter depression?

Abstract

OBJECTIVES: Surveys on winter depression in Iceland indicate a significantly lower prevalence rate of winter SAD than expected according to Iceland's latitude. Research into daylight availability in Iceland failed to reveal factors contributing to higher average daylight availability than predicted by latitude. In view of the well-known healing effects of bright light treatment, we propose that properties of daylight other than daylight availability may ease the symptoms of winter depression. METHOD: We analysed the spectral composition of daylight in Iceland as expressed by its chromaticity and assessed its seasonal and diurnal variations. The colorimetric properties of daylight during the year 1998 are dealt with in detail. Perception of daylight is modelled, applying the chromaticity model of MacLeod and Boynton along with environmental data on spectral irradiance recorded on location at 64 degrees 8.8' N and 21 degrees 55.8' W in Reykjavik, Iceland, and recently published data on cone fundamentals by Stockman and Sharpe. RESULTS: The main finding is that blue hue dominates the colour of the sky, with high correlated colour temperature, without significant seasonal variations. Diurnal variations are, however, observed. Furthermore, significant deviation from 'standard' sky is detected. CONCLUSIONS: It is not known whether the observed chromaticity of daylight is a significant factor in explaining the unexpectedly low prevalence rate of seasonal affective disorder in Iceland.