Feeling blue (Image: Elemental Studios/Alamy)

WE’RE all happier on sunny days, but why? It seems that light taps directly into brain areas that process emotion – good and bad.

Although light is used to treat mood disorders, we don’t understand how this works. While rods and cones in the eye process visible light, a third type of photoreceptor, particularly sensitive to blue light, mediates non-visual responses such as sleep cycles and alertness. So light may make us feel better because it helps regulate circadian rhythms.

Gilles Vandewalle at the University of Liège, Belgium, and colleagues wondered whether this pathway directly affects our emotional state too. To find out, they scanned the brains of volunteers exposed to green or blue light while a neutral or angry voice recited meaningless words. As expected, brain areas responsible for processing emotion responded more strongly to the angry voice, but this effect was amplified by blue light (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1010180107).

Vandewalle suggests blue light is likely to amplify emotions in both directions.