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10 March 2014

A burst of orange light wakes up our ‘circadian eye’

A burst of light could reset your body clock
A burst of light could reset your body clock

(Image: VISUM/Rex)

Feeling dopey? Refresh your “circadian eye” with a burst of orange light.

Light is a powerful wake-up call, enhancing alertness and activity. Its effect is controlled by a group of photoreceptor cells in the eyeball that make the light-sensing pigment melanopsin. These cells, which work separately to the rods and cones needed for vision, are thought to help reset animals’ body clocks – or circadian rhythms. Studies with people who are blind suggest this also happens in humans, although the evidence isn’t conclusive.

To find out how melanopsin wakes up the brain, Gilles Vandewalle at the University of Liege, Belgium, and his team gave 16 people a 10-minute blast of blue or orange light while they performed a memory test in an fMRI scanner. They were then blindfolded for 70 minutes, before being retested under a green light.


People initially exposed to orange light had greater brain activity in several regions related to alertness and cognition when they were retested, compared with those pre-exposed to blue light.

Light switch

Vandewalle thinks that melanopsin is acting as a kind of switch, sending different signals to the brain depending on its state. Orange light, which has the longer wavelength, is known to make the pigment more light-sensitive, but blue light has the opposite effect. Green light lies somewhere in the middle. The findings suggest that pre-exposure to orange light pushes the balance towards the more light-sensitive form of melanopsin, enhancing the response in the brain.

“We knew that light had a non-visual impact on cognitive brain responses but the photoreceptors involved were not definitely established” says Vandewalle. “Our findings constitute compelling evidence in favour of a cognitive role for melanopsin,” he says, adding that more thought should be given to the impact of different colours of light in environments such as schools.

Stuart Peirson from the University of Oxford’s Nuffield Laboratory of Ophthalmology says the findings are intriguing but he points out that several previous studies have suggested that blue light enhances alertness and is more effective at resetting circadian rhythms. “More work is clearly needed to understand the differences between these results,” he says.

Journal reference: PNAS, DOI: 10.1073/pnas.1320005111