It's generally accepted that the amount of ambient light we're exposed to during the day and night helps us keep our circadian clocks in check.
But new research suggests that the colour produced by this light might actually be more important when it comes to accurately predicting the time of day, especially at dusk and dawn.
The results of the study, carried out by biologists at the University of Manchester in the UK, confirm that animals' brains can track changes in colour during twilight, as well as changes in the amount of light, in order to adjust their body clocks and change their behaviour accordingly.
And this ability to discriminate between changes in colour might actually be a more reliable indicator of day's progression into night than the amount or intensity of light, the researchers say.
"This is the first time that we've been able to test the theory that colour affects our body clock in any mammal," said co-author Timothy Brown in a press release.
"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."
Animals use an internal clock to guide their behaviour: when it's dark they know to go to sleep, and when it's light outside, they forage and play and hunt and work, and basically do everything they need to do to survive. And if they're nocturnal, everything is reversed.
But for this internal clock to be of any use, it needs to be synched with external time, which can change depending on the season.
It is fairly well understood that animals use changes in the brightness of ambient light, at different times of day, to help regulate their internal clocks. But prior to this latest study, the researchers said it was unknown whether changes in colour also factored into this process.
To get a better understanding, the researchers looked at the change in light around dawn and dusk across 36 days. In addition to the change in light intensity that occurs as the sun rises and sets, the team found that during twilight, the colour produced by the light was consistently bluer than during the day - a result of certain light frequencies being filtered out when the Sun was below the horizon.
Next, they wanted to calculate the extent to which these changes in colour - or spectral composition - were detectable to the mammalian vision system. So they recorded the electrical activity of the brain clocks of mice when they were shown different visual stimuli designed to test their response to changing colour.
The researchers identified neurons that were activated by the stimuli, and found these neurons to be more sensitive to changes in colour between blue and yellow than to changes in brightness.
In their indoor environment, the scientists then simulated an artificial sky that recreated the daily changes in colour and brightness, as they were measured from a location on campus.
As expected for nocturnal animals, when mice were placed under this artificial sky for several days, the highest body temperatures occurred just after dusk, when the sky turned a darker blue. This indicated to the researchers that their body clocks were working optimally.
When only the brightness of the sky was changed, with no change in colour, the mice became more active before dusk, demonstrating that their body clocks weren't properly aligned to the day-night cycle.
"These data reveal a new sensory mechanism for telling time of day that would be available to any mammalian species capable of chromatic vision," the researchers note.
Their findings have been published in the journal PLOS Biology.
While it's great for enjoying the wonderful world around us, colour vision might also have some deeper function when it comes to keeping our body clocks working smoothly.