The relationship between onsets of depression and sudden drops in solar irradiation

In a retrospective single-case study of a patient (L.S., the first author), who suffers from recurrent depression, we compared the onsets of his depression with meteorological data on light intensity, independently obtained, for the years 1975-1988. During those years there were 24 onsets or worsening of an existing depressive episode. Eleven of these occurred in the months September-December. Seven of these 11 autumn/winter onsets and 12 of the remaining 13 non-autumn/winter onsets coincided with a period of unusually low natural light intensity. Thus, 19 of 24 onsets (79%) were associated with falling light intensity, regardless of season. Some episodes that were not related to falling light intensity were associated with life events. For the years prior to the patient's use of light therapy, there was a statistically significant relationship between falling light and onset of depression. This supports the suggestion that depressive episodes (in certain individuals) may be triggered by unusually dull weather, regardless of season.