Color of Light Strongly Influences the Body’s Internal Clock, University of Manchester Research Shows
Researchers at The University of Manchester have shown that the color of light plays a major role in how the body’s circadian clock determines time of day.
Overview
For the first time, scientists have tested how color—independent of simple brightness—can signal the time of day to the mammalian circadian system. Their findings indicate that colour cues, particularly the blue–yellow shift that occurs during twilight, provide a more reliable indicator of dawn and dusk than changes in light intensity alone. This discovery deepens our understanding of how animals, including humans, align physiology and behaviour with the solar cycle.
What the Study Investigated
The research focused on changes in natural light at dawn and dusk. While it is well known that light intensity varies dramatically during those periods, the team measured the spectral composition of daylight and found a consistent pattern: during twilight the light spectrum tends to shift toward shorter (bluer) wavelengths compared with daytime. The investigators asked whether that spectral shift could be used by the circadian system as a time-of-day signal.
Methods and Key Findings
To test this idea, researchers combined environmental light measurements with laboratory experiments on mice. First, they recorded the changing spectral composition atop the University’s Pariser Building across daily cycles to capture realistic dawn and dusk light profiles. Using those measurements, they constructed a controlled artificial sky capable of reproducing daily changes in both colour (spectral composition) and brightness (irradiance).
Electrophysiological recordings from neurons in the mouse suprachiasmatic nucleus (SCN)—the central circadian clock in mammals—revealed that many clock neurons respond selectively to changes in colour along a blue–yellow axis. These neurons exhibited cone-dependent spectral opponency, meaning they compare inputs from different photoreceptor types to detect spectral shifts. In many cells, responses to spectral changes were stronger than responses to changes in brightness alone.
In behavioural experiments, mice were housed under the artificial sky for several days while researchers measured core body temperature, a robust indicator of circadian state. For these nocturnal animals, the highest body temperatures occurred shortly after nightfall when the simulated sky shifted to a darker blue, indicating proper alignment of their internal clock to the external day–night cycle. By contrast, when the sky’s brightness was varied without accompanying spectral changes, the mice became active earlier than expected before dusk, demonstrating a misalignment of their circadian timing.
Implications and Applications
These results reveal that spectral changes occurring during twilight provide essential timing information for circadian entrainment. As Dr Timothy Brown of the Faculty of Life Sciences, who led the study, explained: separating the effects of colour from brightness has historically been difficult, but new experimental and psychophysical approaches made it possible to show that colour independently contributes to time-of-day signalling. Because the underlying mechanisms depend on cone-based colour discrimination, the findings are relevant to any mammalian species capable of chromatic vision.
The researchers note potential practical applications: if colour cues are powerful regulators of human circadian timing, tailored lighting that manipulates spectral composition could help shift workers adjust schedules, ease jet lag for travellers, or improve general sleep health by better aligning artificial lighting with biological time cues.
Funding: This study was supported by the Biotechnology and Biological Sciences Research Council.
Source: Morwenna Grills, University of Manchester
Image credit: David Gennard, The University of Manchester
Original research: The study is published in PLOS Biology under the title “Colour As a Signal for Entraining the Mammalian Circadian Clock” by Lauren Walmsley, Lydia Hanna, Josh Mouland, Franck Martial, Alexander West, Andrew R. Smedley, David A. Bechtold, Ann R. Webb, Robert J. Lucas, and Timothy M. Brown. Published online April 17, 2015.
Abstract (Summary)
Twilight involves changes in both the amount (irradiance) and the quality (colour) of light. Animals use irradiance variation to synchronize internal circadian clocks with the solar cycle, but whether colour changes also contribute was previously unclear. Environmental measurements indicate that blue–yellow colour discrimination offers a more reliable cue for tracking twilight progression than irradiance alone. Electrophysiological recordings show that neurons within the mouse suprachiasmatic nucleus possess the cone-dependent spectral opponency needed to detect these colour shifts. Some clock neurons are highly sensitive to spectral changes during twilight, and this spectral input shapes their responses to irradiance. Behavioural experiments using simulated dawn and dusk transitions confirm that spectral changes are necessary for proper circadian alignment under natural-like conditions. Together, these findings reveal a previously unrecognized sensory mechanism—chromatic variation—that mammals can use to tell the time of day.