Bright light therapy (BLT) is widely accepted as first-line treatment of SAD. However, the mechanism of action of BLT in the treatment of SAD is still widely unknown. We have challenged the existing paradigm that light treatment would only be mediated through the eyes. In our pilot uncontrolled 4-week transcranial bright light study in SAD, 10/13 SAD subjects achieved remission defined by a 17-item Hamilton Depression Rating Scale (HAMD-17) sum score <= 7; and 12/13 SAD subjects achieved both >= 50% reduction and remission (sum score <= 7) in their self-rated 21-item Beck Depression Inventory (BDI-21) sum scores. In another 4-week study, 89 SAD subjects received 12-min daily doses of photic energy in three different randomly divided groups (1, 4, 9 lumen). When using BDI-21 sum score <=10 and >= 50% reduction in sum score as a cut-off point, remission rates were over 70% in all study groups. In immunohistochemical assessments of brain samples of human cadavers, we have found that e.g., blue light-sensitive melanopsin is widely distributed in human brain. Further, in functional magnetic resonance imaging research branch, we have found that in SAD, the connectivity has increased in visual and sensorimotor networks in human brain. Furthermore, in the corresponding networks, light (via ear canals) sensitivity was found in healthy subjects. Finally, we have investigated also physiological effects of light to find out the mechanism of action of BLT: In controlled studies, light via ear canals has shown to decrease motor time to visual warning signal and visual recognition of masked character in healthy athletes and university students, respectively.