Prof. Manuel Spitschan,
DLA member
Dr. Christine Blume,
DLA Member
1 February 2024
How does the human biological clock respond to the colours of twilight?
Dusk and dawn are key times for biology, providing a signal to organisms that it is the end or beginning of the day. As the sun rises and sets, the spectrum of light in the environment undergoes distinct changes. Not only does the overall intensity of light decrease – there are striking changes in the colour of light changing from blue to yellow or vice versa. Human eyes are set up to detect these changes. A recently published study has now taken a close look whether calibrated changes along the blue-yellow dimension also affect our biological clock.
A study in mice from 2019 reported that effects on the biological clock were stronger to constant light that was yellow and bright than to light that was blue and dimmer. Importantly, the lights did not differ regarding their effect on specialized light-sensitive ganglion cells, which are known to play a key role in the effects of light on the circadian system.
We set out to examine whether this is also the case in humans, that is, whether light colour encoded by the cones influences the biological clock in addition to the light-sensitive ganglion cells. In a laboratory study with 16 participants, we presented lights that were varying in colour and brightness over time. Specifically, the light was slowly flickering and yellow-bright or blue-dim light was presented on top of a white background. These lights, which were designed to examine whether the cones in the eye contribute to setting the biological clock and thus keep the effects on the light-sensitive ganglion cells constant, produced comparable responses in our participants.
Our results provide strong evidence that it is not the colour of twilight that drives the biological clock, emphasizing the role of the specialized retinal ganglion cells. These cells, which were discovered only in the early 2000s, are sensitive to short-wavelength light between 460 and 490 nanometres, and have been found to be the primary drivers of how the circadian clock entrains to the light-dark cycle. Indeed, our findings suggests that the cones in the retina do not provide an additional relevant signal. This has important consequences for light in the built environment, as it simplifies how light designers and planners can evaluate the impact of light on humans.
References
Blume, C., Cajochen, C., Schöllhorn, I., Slawik, H. C., & Spitschan, M. (2023). Effects of calibrated blue-yellow changes in light on the human circadian clock. Nature Human Behaviour. https://doi.org/10.1038/s41562-023-01791-7
Further links
Press release from the University of Basel: https://www.unibas.ch/en/News-Events/News/Uni-Research/Light-colour-is-less-important-for-the-internal-clock-than-originally-thought.html
Comments