University of Basel
Summary Nighttime exposure to short wavelength light can affect the circadian clock, sleep, and alertness. Intrinsically photosensitive retinal ganglion cells that express melanopsin are thought to be the main drivers of these effects. It is unclear whether color-sensitive cones also contribute. Here, using calibrated silent substitution changes in light color along the blue-yellow axis, we investigated whether color vision mechanisms affect the human circadian system and sleep. In a 32.5-h repeated within-subjects protocol, 16 healthy participants were exposed to three different light scenarios for 1 h starting 30 min after their usual bedtime: baseline control condition (93.5 photopic lux). , intermittent blinking (1 Hz, 30 s on–off), bright yellow light (123.5 photopic lux), and intermittently blinking dim blue light (67.0 photopic lux), all calibrated to have the same melanopsin excitation. We found no conclusive evidence of differences between the three lighting conditions with respect to circadian phase delays of melatonin, melatonin suppression, subjective sleepiness, psychomotor vigilance, or sleep. |
Comments
Vision is a complex process. Visual perception of the environment is created by a combination of different wavelengths of light, which are decoded in the brain as colors and brightness. Photoreceptors in the retina first convert light into electrical impulses: with enough light, the cones allow for sharp, detailed and colored vision. Rods only contribute to vision in low light conditions, allowing different shades of gray to be distinguished but leaving vision much less precise. The electrical nerve impulses are eventually transmitted to the retinal ganglion cells and then, through the optic nerve, to the visual cortex of the brain. This brain region processes neural activity into a color image.
What influences the internal clock?
However, ambient light not only allows us to see, but also influences our sleep-wake rhythm . Specialized ganglion cells play an important role in this process, which, like rods and cones, are sensitive to light and react especially strongly to short-wavelength light, about 490 nanometers. If light is composed only of short wavelengths, 440 to 490 nanometers, we perceive it as blue . If short-wavelength light activates the 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 not relevant.
"However, light-sensitive ganglion cells also receive information from the cones. This raises the question of whether the cones, and therefore the color of light, also influence the internal clock. After all, changes The most striking in brightness and color of light occur at sunrise and sunset, marking the beginning and end of the day," says Dr. Christine Blume. At the Center for Chronobiology at the University of Basel she researches the effects of light on humans and she 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 has published their findings in the scientific journal " Nature Human Behavior ".
Light colors in comparison
"A 2019 study in mice suggested that yellowish light has a stronger influence on the internal clock than bluish light," says Christine 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, could also be relevant to the internal clock."
To get to the bottom of this question, the researchers exposed 16 healthy volunteers to a blue or yellowish light stimulus for one hour in the late afternoon, 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 differential and highly controlled manner. However, the stimulation of light-sensitive ganglion cells was the same in all three conditions. Therefore, the differences in the light effect were directly due to the respective stimulation of the cones and ultimately the color of the light.
"This method of light stimulation allows us to experimentally and cleanly separate the properties of light that can influence the effect of light on humans," says Manuel Spitschan, professor of Chronobiology and Health at the Technical University of Munich. who also participated. in the study.
To understand the effects of different light stimuli on the body, in the sleep laboratory the researchers determined whether the participants’ internal clock had 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 beginning of the night. The researchers also asked about their tiredness and tested their responsiveness, which decreases as sleepiness increases.
Ganglion cells are crucial
The conclusion: "We have found no evidence that light color variation in a blue-yellow dimension plays a relevant role in 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, according to which light-sensitive ganglion cells are the most important for the human internal clock," says the scientist.
Manuel Spitschan considers the study an important step towards putting basic research into practice: "Our results show that the most important thing when planning and designing lighting is probably to take into account the effect of light on cells light-sensitive ganglion cells. The cones and, therefore, the color, play a very secondary role."
It remains to be seen whether the color of light also influences sleep if the parameters change and, for example, the duration of light exposure is prolonged or occurs 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 from smartphone and tablet screens affects biological rhythms and sleep. Therefore, it is recommended to put away the mobile phone early in the evening or at least use the night shift mode, which reduces the proportions of short-wavelength light and takes on a slightly yellowish appearance. Christine Blume confirms this. However, yellowing adjustment is a by-product that could be avoided. "Technologically it is possible to reduce the short wavelength ratios even without adjusting the display color, but this has not yet been implemented in commercial mobile phone displays," says the sleep researcher.
Final message In summary, we found no conclusive evidence for an effect of calibrated changes in light color along the blue-yellow axis with constant melanopic arousal on the human circadian system, psychomotor vigilance, drowsiness, or sleep (i.e. , the latency to 10 minutes of continuous sleep). From a more practical perspective, it appears that the human circadian clock is relatively insensitive to changes in light color toward warmer color temperatures in the face of constant melanopic arousal. Smartphones and other displays with night shift modes generally change color and jointly reduce melanopic arousal, and our study provides evidence that any effects observed in night shift mode may be due to reduced arousal. melanopic. As a large body of literature convincingly suggests that the proportions of short-wavelength light should be reduced at night to avoid decreased drowsiness and a phase delay, we recommend users of devices with backlit displays. (i.e. smartphones, tablets and computer screens) to use software or integrated applications such as f.lux in the evenings and at night. In the future, technology companies could also choose to use metameric light that allows short wavelength ratios to be reduced without changing the perceived color. Recently, Schöllhorn and colleagues demonstrated that low-melanopsin light can mitigate the unwanted effects of screen use at night, confirming the major role of melanopsin photoreception in shaping our circadian system by light. |
