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Circadian Rhythms Respond to Color of Daylight, Not Intensity

Though sleep experts have long known that the stability and establishment of our biological, or circadian, rhythms are completely reliant on our exposure to daylight and the rising and setting of the sun, little has been understood about exactly what it is that light does or what factor in daylight serves as the catalyst for our response. But now, a study published in the journal PLOS Biology has found that it is specifically the color of the light that we see, and not its brightness or intensity, that tells our bodies when it is time to wake up and when to go to sleep. As the color of the sky changes in the light of dawn or near sunset, the body recognizes those signals.

The study’s researchers were from the University of Manchester in England, and they began by working with laboratory mice, recording electrical activity within the animals’ bodies as they went about their daily activity under artificial lights programmed to go through transitions that duplicated the appearance of dawn and dusk. The quickly found that the animal’s cells were responding to changes in light rather than to the intensity or brightness of the light. According to researcher Timothy Brown, “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.”

Upon noting the animal’s physiologic responses, the researchers moved to a more elaborate recreation of the sky’s color spectrum that provided an artificial arc from dawn to night. The researchers observed the animals over a period of several days, recording changes in body temperature which are generally considered to be a part of the body’s circadian rhythms. They found that after the artificial sky recreated night fall and the sky turned a darker blue, the animals’ body temperatures reached their highest point, a biomarker for the body clock working exactly as it is supposed to. Alternatively mice were also exposed to adjustments in the artificial sky that only impacted the brightness of the light, and these modifications evoked no change in body temperature – in fact, the mice became more active in response to the brightness, a strong indication that the animals’ circadian rhythms were out of sync with where they were supposed to be in the night time.

According to Brown, “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 travelers wanting to minimize jet lag.”



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