Sensitivity to the reduction in natural daylight in autumn/winter is generally accepted as the main trigger of seasonal depression (seasonal affective disorder).  Light therapy is one treatment option used for such patients. In this study, researchers examined the relationship between exposure to light of different wavelengths, brain emotional stimuli processing (using functional magnetic resonance imaging) and retinal light sensitivity (which was evaluated by electroretinography). They found that blue light enhanced responses to auditory emotional stimuli in the posterior hypothalamus in patients with seasonal affective disorder (SAD), whereas green light decreased these responses. These effects of blue and green light were not observed in healthy control subjects, despite similar retinal sensitivity in SAD and control subjects. They suggest that the posterior hypothalamus may be one brain area involved in seasonal affective disorder, which involves a distinctive response to light and altered emotional responses.

Vandewalle G, Hebert M, Beaulieu C, Richard L, Daneault V, Garon ML, Leblanc J, Grandjean D, Maquet P, Schwartz S, Dumont M, Doyon J, Carrier J: Abnormal Hypothalamic Response to Light in Seasonal Affective Disorder. Biol Psychiatry 2011 Aug 4. [Epub ahead of print].

http://www.ncbi.nlm.nih.gov/pubmed/21820647