Medical hypotheses
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Bright light therapy (BLT) is widely accepted as first-line treatment of seasonal affective disorder (SAD). However, the mechanism of action of BLT is still widely unknown. On the other hand, in mammals, light penetrates the skull bone and reaches the brain, and extra ocular transcranial phototransduction has physiological influences such as changed reproductive cycles and increased brain serotonin levels. ⋯ In conclusion, it is hard to believe that our findings could be explained solely by placebo effect. Consequently, the basic assumptions underlying extraocular photoreception in humans deserve to be reconsidered. Given that a proper placebo treatment can be implemented via ear canals, further investigations with randomized placebo-controlled and/or dose-finding study designs regarding the extraocular transcranial bright light in the treatment of SAD are called for.