Daylight availability: a poor predictor of depression in Iceland

To test the hypothesis that the unexpectedly low prevalence of winter depression in Iceland is explained by Icelanders enjoying more daylight, during the winter months, than allocated to them by latitude.

A conventional photometer was applied to measure illuminance on a horizontal surface at 64 degrees 8.8' N and 21 degrees 55.8' W every minute throughout the year. The illuminance thus measured was compared with computed illuminance, based on theoretical upper bounds.

Daylight availability proved to be, on average, 60% of the theoretical upper bounds derived using clear sky conditions. Snow cover did not, on average, cause a significant increase in daylight availability. Great variability was observed in illuminance from day to day, as well as within days.

Average daylight availability does not explain the lower than expected prevalence of winter depression in Iceland. The great variability in illuminance might, however, affect the expression of winter depression, as could daylight quality and genetic factors.