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 colors 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 colored vision. Rods only contribute to vision in low light conditions allowing for different shades of grey to be distinguished but leaving vision much less precise. The electrical nerve impulses are finally transmitted to ganglion cells in the retina and then via the optic nerve to the visual cortex in the brain. This region of the brain processes the neural activity into a colored image.

What influences the internal clock?

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 color is not relevant.

“However, the light-sensitive ganglion cells also receive information from the cones. This raises the question of whether the cones, and thereby the light color, also influence the internal clock. After all, the most striking changes in brightness and light color occur at sunrise and sunset, marking the beginning and end of a day,” says Dr. Christine Blume. At the Centre for Chronobiology of the University of Basel, she investigates the effects of light on humans and is the first author of a study investigating the effects of different light colors on the internal clock and sleep. The team of researchers from the University of Basel and the TUM has now published its findings in the scientific journal “Nature Human Behaviour”.

Light colors in comparison

“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 color 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 color-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 color of the light.

Manuel Spitschan sees the study as an important step towards putting basic research into practice: “Our findings show that it is probably most important to take into account the effect of light on the light-sensitive ganglion cells when planning and designing lighting. The cones and therefore the color play a very subordinate role.”

It remains to be seen whether the color 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.