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In these studies, volunteers with normal vision were given a variety of tests while exposed to bright light during the day. Their performance in visual searches, mathematics, logical reasoning and reaction time all improved with exposure to bright light (Trends in Cognitive Sciences, vol 13, p 429).

This appears to be because light boosts alertness. In another study, volunteers had their brains scanned as they performed a short-term memory task while exposed to either violet, blue or green light. The scans revealed that after just a few seconds of light exposure an area of the brain stem known to play a role in alertness became more active (PLoS One, vol 2, p 1247). Blue light was the most potent. Similarly, in simple reaction tasks, exposure to blue light is more effective in sustaining cognitive performance than green light (Sleep, vol 29, p 161).

These effects are probably mediated by a recently discovered pigment in the retina called melanopsin, which is not involved in vision. Melanopsin absorbs pale blue light better than other wavelengths, which is not surprising as natural light contains a lot of blue. But exactly how it boosts cognition remains unclear.

"No doubt further research will expand our understanding of the characteristics of the light environment that are required to optimise brain function," says Gilles Vandewalle, a neuroscientist at the University of Montreal, Canada