A sunlight deficiency, as is experienced at our latitude in winter, induces a seasonal affective disorder (SAD) or winter depression in some people. First line of treatment of this form of depression is bright-light therapy, as a type of substitution therapy. SAD is treated with similar success using antidepressant drugs like hypericum (St. John's wort). Phototherapy also has an antidepressive effect on non-seasonal depressions, albeit not quite as pronounced. When using phototherapy light is transformed into electric impulses in the retina. These impulses are transmitted to the hypothalamus and the central nervous structures controlling metabolism, hormones and the circadian rhythms. These processes are governed by very complex regulatory mechanisms. Feedback loop mechanisms induce constant adaptation of the photosensitivity of the retinal photoreceptors while also controlling the central nervous structures. New findings on the interaction and interweaving of depression, light, metabolism, hormones and circadian rhythms support the following hypothesis: The photodynamic impact of hypericum magnifies the effect of normal light, as if the patient were subject to continuous light therapy. The photosensitizing effect of hypericum is thus not only of interest as an undesirable side-effect but also for its therapeutic effects.