Light at night is considered bad for sleep. However, does the color of the light matter? Researchers from the University of Basel and the Technical University of Munich (TUM) compared the effect 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 colors and brightness in the brain. The photoreceptors in the retina first convert light into electrical impulses: with enough light, the cones enable sharp, detailed and color vision. Rods only contribute to low-light vision, allowing different shades of gray to be distinguished, but leave vision much less accurate. Electrical nerve impulses are eventually transmitted to ganglion cells in the retina and then via the optic nerve to the visual cortex of the brain. This area of the brain processes neural activity into a color image.
What affects the internal clock?
Ambient light, however, not only allows us to see, but also affects our sleep-wake rhythm. Importantly involved in this process are specialized ganglion cells, which – like rods and cones – are sensitive to light and react particularly strongly to short-wavelength light at a wavelength of about 490 nanometers. If the light consists exclusively of short wavelengths from 440 to 490 nanometers, we perceive it as blue. If the short-wavelength light activates the ganglion cells, they signal the internal clock that it is daytime. The determining factor here is how intense the light is per wavelength. 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 therefore the color of the light also affect the internal clock. After all, the most striking changes in the brightness and color of the light occur in sunrise and sunset, marking the beginning and end of a day,” says Dr. Christine Bloom. At the University of Basel’s Center for Chronobiology, she researches the effects of light on humans and is the first author of a study investigating the effects of different of bright colors in the internal clock and in sleep The team of researchers from the University of Basel and TUM have now published their findings in the journal “Nature Human Behavior”.
Light colors in comparison
“A study in mice in 2019 showed that yellowish light has a stronger effect on the internal clock than blue light,” says Christine Blume. In humans, the primary effect of light on the internal clock and sleep is probably mediated through photosensitive ganglion cells. “However, there is reason to believe that the color of light, which is encoded by the cones, could also be relevant to the internal clock.”
To find out, the researchers exposed 16 healthy volunteers to a bluish or yellowish light stimulus for one hour late at night, as well as a white light stimulus as a control condition. The light stimuli were designed to differentially activate color-sensitive cones in the retina in a very controlled manner. However, photosensitive ganglion cell excitation was the same in all three conditions. Therefore, the differences in the effect of the light were directly due to the respective stimulation of the cones and ultimately to the color of the light.
“This method of light stimulation allows us to tease apart the properties of light that may play a role in how light affects people in a purely experimental way,” says Manuel Spitschan, professor of chronobiology and health at the Technical University of Munich, who also participated. in the study.
In order to understand the effects of different light stimuli on the body, in the sleep lab the researchers determined whether the participants’ internal clock had changed depending on the color of the light. In addition, they assessed how long it took the volunteers to fall asleep and how deep their sleep was at the beginning of the night. The researchers also investigated their fatigue and tested their ability to react, which decreases with increasing sleepiness.
Ganglion cells are vital
The bottom line: “We found no evidence that the variation of light color along a blue-yellow dimension plays an important role for the human internal clock or sleep,” says Christine Blume. This contrasts with the results of the mouse study mentioned above. “Rather, our results support the findings of many other studies that light-sensitive ganglion cells are the most important for the human internal clock,” says the scientist.
Manuel Spitschan sees the study as an important step towards implementing basic research: “Our findings show that it is probably more important to consider the effect of light on light-sensitive ganglion cells when planning and designing lighting. so color plays a very secondary role.”
It remains to be seen whether the color of the light also has no effect on sleep, if the parameters are changed and, for example, the duration of light exposure is extended or takes place at a different time. Subsequent studies should answer questions like these.
Night mode on screens — useful or not?
We often hear that the short-wavelength component of light from smartphone and tablet screens affects biological rhythms and sleep. Therefore, the recommendation is to put your mobile away in the early evening or at least use the night shift mode, which reduces the proportions of short-wavelength light and appears slightly yellowish. Christine Blume confirms it. However, yellowing is a by-product that could be avoided. “Technologically, it is possible to reduce short-wavelength ratios even without color adjustment of the display, however this has not yet been implemented in commercial mobile phone displays,” says the sleep researcher.