A sunset may not be just a pretty thing to watch- it could be affecting our health, research suggests.
Colour has a greater impact on circadian rhythm - the day/night clock system that regulates bodily functions - than light intensity, scientists have found.
In a series of experiments, researchers created an "artificial sky" which reproduced daily changes in colour and brightness.
Mice exposed to the "sky" for several days were found to be most sensitive to colour.
Other tests conducted showed similar results, where the body clocks of mice were more affected by changes in colour between blue and yellow than light levels.
Dr Timothy Brown, from the Faculty of Life Sciences at the University of Manchester, said: "This is the first time that we've been able to test the theory that colour affects the body clock in mammals.
"It has always been very hard to separate the change in colour to the change in brightness, but using new experimental tools and a psychophysics approach we were successful.
"What's exciting about our research is that the same findings can be applied to humans. So in theory, colour could be used to manipulate our clock, which could be useful for shift workers or travellers wanting to minimise jet lag."
The scientists, whose findings appear in the online journal Public Library of Science Biology, began by looking at changes in light around dawn and dusk to see whether colour could be used to determine time of day.
They made the discovery that light is reliably bluer during twilight.
The researchers then recorded body clock electrical activity in mice shown different visual stimuli, and constructed the artificial sky using measurements of colour changes taken from the top of the university's Pariser Building.
While the mice were exposed to the artificial sky, their body temperature was recorded. As expected for nocturnal animals, they were warmest just after sunset - showing that their body clocks were working normally.
Peak temperature occurred later when twilight was recreated using the correct colour as well as intensity of light.
The scientists wrote: "Our data .. indicates that most mammals could use colour to provide additional information about sun position, above that available from simply measuring irradiance ..
"Alongside our present data, it appears .. that the use of colour as an indicator of time of day is an evolutionarily conserved strategy, perhaps even representing the original purpose of colour vision ..
"Based on the widespread capacity for colour vision among mammals, we suggest related mechanisms are likely to be broadly applicable across many mammalian species."
