Light Color and Internal Clock

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. 

Ambient light however does not only allow us to see, it also influences our sleep-wake rhythm. 

Specialised ganglion cells are significantly involved in this process, which — like the cones and rods — are sensitive to light and react particularly strongly to short-wavelength light at a wavelength of around 490 nanometres. 

If light consists solely of short wavelengths of 440 to 490 nanometres, we perceive it as blue. If short-wavelength light activates the ganglion cells, they signal to the internal clock that it is daytime. The decisive factor here is how intense the light is per wavelength; the perceived colour is not relevant.

“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. 

However, the stimulation of the light-sensitive ganglion cells was the same in all three conditions. Differences in the effect of the light were therefore directly attributable to the respective stimulation of the cones and ultimately the colour of the light.

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.

It remains to be seen whether the colour of the light also has no effect on sleep if the parameters change and, for example, the duration of the light exposure is extended or takes place at a different time. Follow-up studies should answer questions like these.


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

University of Basel. (2023, December 22). Light color is less important for the internal clock than originally thought. ScienceDaily. Retrieved December 22, 2023 from

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