The beautiful color of a sunset might be more than just a pretty picture. It could be a signal to our bodies that it’s time to reset our internal clock, the biological ticktock that governs everything from sleep patterns to digestion. That’s the implication of a new study in mice that shows these small rodents use light’s changing color to set their own clocks, a finding that researchers expect will hold for humans, too.
“I think this work opens up how we're just starting to scratch the surface and look at the environmental adaptations of clocks,” says Carrie Partch, a biochemist at the University of California, Santa Cruz, who was not involved in the new study.
Scientists have long known about the role light plays in governing circadian rhythms, which synchronize life’s ebb and flow with the 24-hour day. But they weren’t sure how different properties of light, such as color and brightness, contributed to winding up that clock. “As a sort of common sense notion people have assumed that the clock somehow measures the amount of light in the outside world,” says Tim Brown, a neuroscientist at the University of Manchester in the United Kingdom and an author of the new study. “Our idea was that it might be doing something more sophisticated than that.”
To find out, Brown and his colleagues targeted an area in the brain called the suprachiasmatic nucleus, or SCN, a region common to all vertebrates. It’s where the body keeps time using chemical and electrical rhythms that last, on average, 24 hours. The team wanted to know if color signals sent from the eyes reached the SCN and whether that information affected the timing of the clock.
Brown and his colleagues measured the electrical chatter of SCN neurons as they showed mice different intensities and colors of light. At least a quarter of the neurons they measured responded strongly to changes in color, especially to the shorter wavelength blue light that dominates skies after dusk. This meant that the signal was getting through, but it did not demonstrate a measurable effect on the mice’s internal clocks. So the team constructed an “artificial sky”—a bank of differently colored LEDs behind a diffusive screen—above caged mice and simulated day and night with and without color changes. Mice are nocturnal, with a body temperature that peaks during the night. When natural color changes were missing from the artificial sky, their clocks got confused—the peak temperature arrived about 30 minutes earlier than under more natural conditions.
To be sure that this shift in temperature was indeed due to a change in the clock, the researchers examined slices from the SCN of mice involved in the artificial sky experiment. “One of the cool things about the clock,” Brown says, is that “when you take it out of an animal and put it in a dish, the cells continue to fire.” By measuring the firing rates in a particular slice, the researchers could estimate if the clock was running fast or slow. Cells from mice that didn't see color variation lagged behind those that did, a confirmation that the shift in peak body temperature was due to the clock, the team reports today in PLOS Biology.
Brown sees potential human applications in this work. "What this opens up is the possibility for enhancing existing ways of treating jet lag or things like seasonal depression disorder,” he says. One method for treating jet lag is a light box, which immerses a traveler in bright light to trick his or her clock. Adding color to that light might give better results, Brown says. The new finding may even change our understanding of why color vision evolved. The researchers suggest it may have been a better way for animals to set their clocks in a world where clouds could reduce the brightness of light but still let color shine through.