Body's Internal Clock Depends on Color of Light

It is well known that brightness drives our circadian rhythms, but for the first time new research shows that the body's internal clock also depends on the color of light to measure the time of day.

"This is the first time that we've been able to test the theory that color affects the body clock in mammals. It has always been very hard to separate the change in color to the change in brightness but using new experimental tools and a psychophysics approach we were successful," lead author Dr. Timothy Brown, from The University of Manchester, said in a statement.

The researchers looked at the change in light around dawn and dusk to analyze whether color could be used to determine time of day. Besides the well-known changes in light intensity that occur as the Sun rises and sets, the scientists found that during twilight, light is reliably bluer than during the day.

Using mice as study subjects, they recorded electrical activity from the body clock when the animals were shown different visual stimuli. The team discovered that the neurons of the brain were much more sensitive to changes in color between yellow and blue, rather than to changes in brightness.

In line with these initial findings, the scientists then created an artificial sky that could mimic the daily changes in color and brightness during the day. When the mice were placed under the artificial sky for several days, the highest body temperatures were recorded after dusk - when the sky turned dark blue. Because mice are nocturnal animals, these results suggest that their body clocks are at its optimal function during this time.

However, when the researchers only changed the brightness of the sky, and not the color, the body clock didn't work properly in mice, and they were more active before dusk.

This study is the first to demonstrate that color may provide a more reliable way of telling the time than measuring brightness, which could prove to be useful in the real world.

"What's exciting about our research is that the same findings can be applied to humans. So in theory color could be used to manipulate our clock, which could be useful for shift workers or travellers wanting to minimize jet lag," Brown added.

The results were published in the journal PLOS Biology.

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