University of Manchester at sunset. Credit: David Gennard, The University of Manchester
A study from The University of Manchester (U.K.) shows that changes in light color, not just in light brightness, affect the accuracy of mammalian body clocks (PLoS Biol., doi: 10.1371/journal.pbio.1002127). According to the study’s authors, this is the first time the impact of color on circadian rhythms has been tested and measured. Team lead Timothy Brown says their findings can be applied to humans to “manipulate our clock, which could be useful for shift workers or travelers wanting to minimize jet lag.”
Mammals have internal “clocks” that help align behavior and body functions to appropriate times of day. For this clock to be useful, it must be synchronized to the environment. The retino-hypothalamic projection (RHP) in the eye brings visual cues to the brain’s clock, which is located in the suprachiasmatic nuclei (SCN). The RHP contains photoreceptive cells that inform the SCN about changes in light intensity. But the same pathway also contains cones, which raises the possibility that the body might also use changes in color to tell time.
Brown and his colleagues aimed to directly test that speculation—or, more specifically, the notion that changes in light color play a role in synchronizing biological clocks to the solar cycle in animals with color vision. To do so, the scientists first recorded the spectral composition and brightness in the sky above their university for 36 days. In addition to the expected light and dark patterns, they found that the spectral composition patterns—specifically the variations in blue and yellow light—were more predictive of the sun’s position than light intensity.
Next, to see if mammalian SCNs show an electrophysiological response to changes in color, the researchers recorded the SCN extracellular activity in mice exposed to different visual stimuli. They found that nerve cells in SCN can track changes in color from blue to yellow—color changes that occur during sunrises and sunsets.
To confirm that these electrophysiological responses occur during day-night cycles, the researchers placed mice in environments that simulated the dawn/dusk brightness and color transitions they observed in the Manchester sky. When the mice experienced the sky simulation with changes in both color and brightness, the researchers saw more accurate nocturnal behavior (e.g., changes in body temperature and activity). While they also observed nocturnal behavior in mice in the brightness-only simulation, the timing wasn’t as aligned with the day-night cycle. These results suggest that changes in light color allow for more accurate timekeeping than changes in brightness alone.
The authors admit that their simulations lack the variations in light and color that happen with cloud cover and other weather conditions, but they believe that their demonstration shows that color has an impact on the phasing of body clocks in mammals with color vision. They also suggest that the use of color as a time indicator may represent the original evolutionary purpose of color vision.