Light in the evening is considered bad for sleep. But does it matter what color the light is? In a study, researchers have now compared how different light colors affect our tiredness and our sleep rhythm. In contrast to the results of previous studies, blue light did not make people more awake than yellow light – at least when the intensity of the light was adjusted so that the relevant cells on the retina were activated to a similar extent.
When it gets dark in the evening, the pineal gland in our brain releases the sleep hormone melatonin. However, light suppresses this process by activating the so-called ganglion cells in our retina. These then send signals to the brain that prepare us for “day”. They react particularly strongly to short-wave light with a wavelength of around 490 nanometers. We perceive this wavelength as blue with the help of our visual cells, the cones, which specialize in colors. However, it was still unclear whether the color we perceive plays a role in fatigue.
Brightness adjusted
A team led by Christine Blume from the University of Basel has now investigated this question. “The light-sensitive ganglion cells are not only activated by light themselves, but also receive information from the cones,” she explains. “Therefore, one can ask whether the cones and therefore the color also play a role in the internal clock.” For the study, the team exposed 16 test subjects to blue, yellow or, as a control condition, white light for an hour late in the evening.
In order to keep the activation of the ganglion cells constant and only measure the influence of the light color, the researchers adjusted the intensity of the light. They presented the blue light dimmed, but the yellow in strong brightness. “This method of light stimulation allows us to cleanly experimentally separate the properties of light that may play a role in how light affects people,” explains co-author Manuel Spitschan from the Technical University of Munich.
Intensity more important than color
Each subject completed the study on different days under each of the lighting conditions. Before, during and after exposure to light, the participants filled out questionnaires about their fatigue and completed tests on their ability to react. The researchers also measured the melatonin concentration using saliva samples, recorded brain waves using an electroencephalogram (EEG) and monitored in the sleep laboratory how long it took the people to fall asleep after the intervention and how deeply they slept.
The result: “We found no evidence that the variation of light color along a blue-yellow dimension plays a relevant role for the human internal clock or sleep,” says Blume. A similar experiment with mice, however, showed that the strong yellow light had a greater influence on the internal clock than the dimmed bluish light. The current study could not confirm this. “In fact, our results support the results of many other studies that the light-sensitive ganglion cells have the greatest importance for the human internal clock,” says Blume.
Implications for practice
Due to the small number of test subjects, the significance of the study is limited. Nevertheless, it provides information that can help to make lighting concepts indoors or for electrical devices such as smartphones, tablets, etc. as sleep-friendly as possible. “Our findings suggest that the effect of light on the light-sensitive ganglion cells should probably be taken into account when planning and designing lighting. The cones and therefore the color play a very minor role,” says Spitschan.
Many smartphones already have a night mode that reduces the amount of short-wave light. The colors on the display shift to yellowish. “Our study suggests that any effects of night mode are due to a reduction in ganglion cell activation,” the team writes. It should therefore be just as effective if the proportion of short-wave light is reduced without causing a noticeable color shift. “This would already be technologically possible, even if it has not yet been implemented in commercial cell phone displays,” says Blume.
Source: Christine Blume (University of Basel) et al., Nature Human Behavior, doi: 10.1038/s41562-023-01791-7