Light color is less important for the internal clock than originally thought

Light in the evening is thought to be bad for sleep. However, does the color of the light play a role? Researchers from the University of Basel and the Technical University of Munich (TUM) compared the influence of different light colors on the human body. The researchers' findings contradict the results of a previous study in mice.

Vision is a complex process. The visual perception of the environment is created by a combination of different wavelengths of light, which are decoded as colours and brightness in the brain. Photoreceptors in the retina first convert the light into electrical impulses: with sufficient light, the cones enable sharp, detailed, and coloured vision.

"A study in mice in 2019 suggested that yellowish light has a stronger influence on the internal clock than blueish light," says Christine Blume. In humans, the main effect of light on the internal clock and sleep is probably mediated via the light-sensitive ganglion cells. "However, there is reason to believe that the colour of light, which is encoded by the cones, could also be relevant for the internal clock."

To get to the bottom of this, the researchers exposed 16 healthy volunteers to a blueish or yellowish light stimulus for one hour in the late evening, as well as a white light stimulus as a control condition. The light stimuli were designed in such a way that they differentially activated the colour-sensitive cones in the retina in a very controlled manner.

"This method of light stimulation allows us to separate the light properties that may play a role in how light effects humans in a clean experimental way," says Manuel Spitschan, Professor of Chronobiology and Health at the Technical University of Munich, who was also involved in the study.

In order to understand the effects of the different light stimuli on the body, in the sleep laboratory the researchers determined whether the internal clock of the participants had changed depending on the colour of the light.

The conclusion: "We found no evidence that the variation of light colour along a blue-yellow dimension plays a relevant role for the human internal clock or sleep," says Christine Blume. This contradicts the results of the mouse study mentioned above. "Rather, our results support the findings of many other studies that the light-sensitive ganglion cells are most important for the human internal clock," says the scientist.

Reference: Christine Blume, Christian Cajochen, Isabel Schöllhorn, Helen C. Slawik, Manuel Spitschan. Effects of calibrated blue–yellow changes in light on the human circadian clock. Nature Human Behaviour, 2023; DOI: 10.1038/s41562-023-01791-7