Editor's Pick: Light at night is considered 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 effects of different colors of light on the human body. The researchers' findings contradict the results of previous studies in mice.
Vision is a complex process. Visual perception of the environment is made up of a combination of different wavelengths of light, which are decoded into color and brightness in the brain. Photoreceptors on the retina first convert light into electrical impulses: with enough light, the cones are able to produce clear, detailed, and colored vision. Rod cells only contribute to vision in low-light conditions, allowing to distinguish between different shades of gray, but making vision less precise. Nerve impulses are eventually transmitted to the ganglion cells of the retina and then to the visual cortex of the brain through the optic nerve. This area of the brain processes neural activity into color images.
However, ambient light not only allows us to see, but also affects our sleep-wake rhythm. Special ganglion cells play an important role in this process, and they are as sensitive to light as cones and rods, and are particularly responsive to short-wavelength light with a wavelength of about 490 nanometers. If the light is a short wavelength of 440 to 490 nanometers, we can perceive the color blue. If short-wavelength light activates ganglion cells, they signal the internal clock for the day. The decisive factor here is the intensity of each wavelength of light;The perceived color is irrelevant.
However, photosensitive ganglion cells also receive information from cones. This raises the question: does the cones, and therefore the light color, also affect the biological clockAfter all, the most significant changes in brightness and color occur at sunrise and sunset, marking the beginning and end of the day. Dr. Christine Blume said. At the Center for Temporal Biology at the University of Basel, she studies the effects of light on humans and is the first author of a study that investigated the effects of different colors of light on internal clocks and sleep. The research team from the University of Basel and the German Technical University published their findings in the scientific journal Nature Human Behavior.
A 2019 study in mice showed that yellow light had a greater effect on the internal clock than blue light. In humans, the main effects of light on the circadian clock and sleep may be mediated by light-sensitive ganglion cells, Christine Blume said. "However, there is reason to believe that the color of the light encoded by the cones may also be related to the internal clock. ”
To figure this out, the researchers had 16 healthy volunteers receive blue or yellow light stimulation for one hour late at night, while white light stimulation was used as a control condition. Photostimuli are designed in such a way that they differentially activate color-sensitive cones on the retina in a very controlled manner. However, in all three cases, the stimulation of photosensitive ganglion cells was the same. Thus, the difference in the effect of light is directly attributable to the stimuli to which the cones are individually subjected, and ultimately to the color of the light.
Manuel Spitschan, a professor of temporal biology and health at the Technical University of Munich, who was also involved in the study, said: "This method of light stimulation allows us to isolate in a clean experimental way the properties of light that may play a role in how light affects humans. ”
To understand the effects of different light stimuli on the body, in the sleep lab, researchers determined whether the participants' internal clocks changed with the color of the light. In addition, they assessed the time it took for the volunteers to fall asleep and how deep they slept at the beginning of the night. The researchers also asked them about their level of fatigue and tested their ability to react, which decreases as drowsiness increases.
"We did not find any evidence that changes in the color of light in the blue-yellow dimension have a correlated effect on the body's internal clock or sleep." This contradicts the results of the mouse study mentioned above, says Christine Blume. "Instead, our results support the findings of many other studies that light-sensitive ganglion cells are most important for the body's internal clock."
Manuel Spitschan sees this research as an important step in putting basic research into practice: "Our findings suggest that considering the effects of light on light-sensitive ganglion cells may be of paramount importance when planning and designing lighting. Cone cells and color play a very minor role. ”
If the parameters change, such as the duration of the light exposure is prolonged or occurs at a different time, it remains to be seen whether the color of the light also has no effect on sleep, and subsequent studies should answer these questions.
We often hear that the short-wave light emitted by smartphone and tablet screens can affect biorhythms and sleep. Therefore, it is advisable to put your phone away earlier in the evening, or at least use the night shift mode, which will reduce the proportion of light at short wavelengths and will look a little yellowish. This is confirmed by Christine Blume. However, yellowish color adjustments are a by-product that can be avoided. "Technically, it is possible to reduce the proportion of short wavelengths even without adjusting the color of the display, but this has not yet been achieved on commercial mobile phone displays. Sleep researchers noted.
*: Nature Human Behavior magazine.
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