Color of light is less important to the internal clock than initially thought, study finds

Vision is a complex process. 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 light into electrical impulses: with enough light, the cones enable sharp, detailed, and colorful vision. The rods only contribute to vision in low light conditions, allowing different shades of gray to be distinguished, but leaving much less precise vision.

Electrical nerve impulses are ultimately transmitted to ganglion cells in the retina, then via the optic nerve to the visual cortex of the brain. This region of the brain transforms neuronal activity into a colorful image.

What influences the internal clock?

However, ambient light not only allows us to see; it also influences our sleep-wake rhythm. Significantly involved in this process are specialized ganglion cells, which, like cones and rods, are sensitive to light and respond particularly strongly to light with a short wavelength of around 490 nanometers.

If the light consists only of short wavelengths of 440 to 490 nanometers, we perceive it as blue. If short-wavelength light activates ganglion cells, they signal the internal clock that it is daytime. The deciding factor here is the intensity of the light per wavelength; the perceived color is irrelevant.

“However, light-sensitive ganglion cells also receive information from the cones. This raises the question of whether the cones, and thus the color of the light, also influence the internal clock. After all, the most striking changes “Brightness and color of light occur at sunrise and sunset, marking the beginning and end of a day,” explains Dr. Christine Blume.

At the Center for Chronobiology at the University of Basel, she studies the effects of light on humans and is the first author of a study investigating the effects of different colors of light on the internal clock and sleep. The team of researchers from the University of Basel and TUM published their results in the journal Nature Human behavior.

Comparison of light colors

“A study in mice in 2019 suggested that yellowish light had a stronger influence on the internal clock than bluish light,” says Dr. Blume. In humans, the main effect of light on the internal clock and sleep is likely mediated by light-sensitive ganglion cells. “However, there is reason to believe that the color of light, encoded by the cones, might also be relevant to the internal clock.”

To get to the bottom of this, the researchers exposed 16 healthy volunteers to a bluish or yellowish light stimulus for an 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 activated the color-sensitive cones of the retina in a very controlled manner.

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

“This method of light stimulation allows us to separate in a clean experimental way the properties of light that may play a role in how light affects humans,” explains Manuel Spitschan, professor of chronobiology and health at the Technical University from Munich, who also participated in the study.

To understand the effects of different light stimuli on the body, the sleep laboratory researchers determined whether the participants’ internal clock changed depending on the color of the light. Additionally, they assessed how long it took the volunteers to fall asleep and how deep their sleep was at the start of the night. The researchers also asked about their fatigue and tested their ability to respond, which decreased with increasing sleepiness.

Ganglion cells are crucial

The conclusion: “We found no evidence that variation in light color along a blue-yellow dimension plays a role relevant to the human internal clock or sleep,” says Dr. Blume. This contradicts the results of the mouse study mentioned above. “Our results rather confirm the findings of many other studies that light-sensitive ganglion cells are the most important for the human internal clock,” explains the scientist.

Manuel Spitschan considers this study an important step towards putting fundamental research into practice. “Our results show that it is probably very important to consider the effect of light on light-sensitive ganglion cells when planning and designing lighting. Cones, and therefore color, play a very secondary role,” he says.

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

Night mode on screens: useful or not?

We often hear that the short wavelength component of light emitted by smartphone and tablet screens affects biological rhythms and sleep. It is therefore recommended to put away your cell phone early in the evening or at least use night mode, which reduces the proportions of short-wavelength light and appears slightly yellowish. Dr. Blume confirms this. However, the yellowish color adjustment is a by-product that could be avoided.

“Technologically, it is possible to reduce the proportions of short wavelengths even without adjusting the screen colors; however, this has not yet been implemented on commercial cell phone screens,” explains the researcher on sleep.