Chronobiology: Half-past blue - how colour tells the time

The environmental cues that set the mammalian body clock include the colour as well as the intensity of daylight, according to research on mice at Manchester University. Light is bluer around dawn and dusk and yellower when the sun is higher in the sky - and cells in the circadian clock respond to this difference.

By analysing the natural light spectrum on a university roof, the scientists were surprised to find that colour was a more accurate indicator of time during twilight than the brightness of daylight. "This most likely reflects the fact that cloud cover can change overall brightness quite dramatically but exerts only relatively minor effects on spectral composition," they write in the journal PLOS Biology.

The researchers then measured electrical activity in the "suprachiasmatic" cells, which control the clock in the brain, and found that many of them were more sensitive to changes in colour between blue and yellow than to changes in brightness.

Next they let the mice live under an "artificial sky", which recreated real diurnal changes in colour and brightness in the laboratory, and followed their body temperature over several days. This reached a peak just after sunset as the sky turned a darker blue, as predicted for nocturnal animals. But when light intensity was changed without any change in colour, the mice became active before dusk, showing that their clock was not aligned properly with the diurnal cycle.

"This is the first time that we've been able to test the theory that colour affects mammals," says Timothy Brown, study leader. "It has always been very hard to separate the change in colour from the change in brightness but we succeeded using new tools and a psychophysics approach."

The findings can be applied to people too: "In theory colour could be used to manipulate our clock, which could be useful for shift workers or travellers wanting to minimise jet lag," says Brown.

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