We exist on a planet revolving around a star. Light streaming from this brilliant sun stimulates our minds and so motivates our behaviors. Now, new research from University of Manchester finds it's not just brightness but color that influences how our internal clocks keep track of time. The hues of the sky, then, help us adjust to the rhythms of the day.

"The two different sources of information are combined such that changes in color will alter the [circadian] clock's response to light of a given intensity," Dr Timothy Brown, faculty of Life Sciences at the University of Manchester, told Medical Daily in an email.

Crayola defines color as “the aspect of things that is caused by differing qualities of light being reflected or emitted by them.” The maker of crayons used by children far and wide explains how light is necessary for us to see — when it shines, some colors bounce off an object, while others are absorbed. Our eyes can only distinguish the reflected colors.

Meanwhile, light is both a particle and a ray — a ray made of electromagnetic waves. Different wavelengths are either visible or invisible to our eyes in the form of color. The longest wavelength that we can see is red, while the shortest is violet. Ultraviolet, a shorter wavelength than violet, cannot be seen by our eyes, while infrared, which is a longer wavelength than red, is also beyond our scope. Whether seen or not, wavelengths can be felt in terms of their influence on our bodies.

“Color stimulates certain moods in us,” said Hans Hofmann, a painter known for the exuberance of his palette. His color field painting, a specific abstract style, attempts to connect with the primal emotions found in ancient myths. “The whole world, as we experience it visually, comes to us through the mystic realm of color,” Hofmann said. “Our entire being is nourished by it.” His belief in the power of color was spot on, as a group of researchers from Manchester demonstrate.

Mystical Color

The researchers wanted to know whether the change in color could be used to determine the time of day between dawn and dusk. First the researchers measured the natural variation in color occurring across multiple days. Then, they calculated the extent to which these color changes are detectable and compared the day-to-day variations in color and to the daily differences in brightness.

Surprisingly, color better predicted sun position (and so hour of the day) than brightness. “Cloud cover can change overall brightness quite dramatically, but exerts only relatively minor effects on spectral composition,” wrote the researchers in their paper. During twilight, in fact, the light is reliably bluer than during the day.

Next, while exposing a group of experimental mice to different visual stimuli, the science team recorded electrical activity in these rodents' suprachiasmatic nuclei — the master clock in the brain that influences circadian rhythm (or daily pattern) of sleep and other behaviors. The electrical activity recorded by the scientists showed the mice to be more sensitive to changes in color than in brightness.

Finally, the researchers constructed an artificial sky that recreated daily changes in color and brightness and placed the mice beneath it for several days. (Mice are able to discriminate color, but they see a much more limited spectrum than humans do.) Their highest body temperatures occurred just after night fell when the sky had turned a dark blue — what would be expected for a nocturnal animal in tune with daily rhythms. In other words, their body clocks worked optimally when provided both color and brightness.

Asked what happens with mammals and most importantly people who are color blind, Brown told Medical Daily, "We found that the clock specifically measures blue level color. Blue color blindness is very rare in humans, but our prediction is that those who lack the ability to distinguish the colour blue may have a harder time synchronising their body clock to the external environment."

Exceptions aside, "our data reveal a new sensory mechanism for estimating time of day," note the authors in their published study. And so we earthly creatures respond not only to the brightness of our star but also to the many colors it produces on this whirling planet.

Source: Walmsley L, Hanna L, Mouland J, et al. Colour As a Signal for Entraining the Mammalian Circadian Clock. PLOS Biology. 2015.

Note: An earlier version of this article lacked comments from Dr. Brown.