Can I counter light with more light?

Sleep: Blue light might be less of a problem than expected

Light in the evening disturbs our sleep. How exactly, however, has been controversial among scientists since they discovered new light sensory receptors at the beginning of the millennium, which have a direct effect on our internal clock. The biological details are tricky, but research has now largely agreed that sleep is particularly difficult for the body if you go to bed directly from a very bright environment. The manufacturers of screens, tablet computers and smartphones have also noticed that blue light is particularly annoying to the eyes and sleep: They have long since offered various blue light filters that are supposed to make the devices easier to use in the evening. Researchers in the journal Current Biology think that these filters are perhaps bypassed reality: according to their new experiments, colder, blue light itself might even be less disturbing than the supposedly lighter shades that are supposed to promote sleep

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The starting point of the research by Timothy Brown and his colleagues from the University of Manchester was astonishment: About the fact that blue light, of all things, should disturb in the evening. After all, in the evening at every sunset in the great outdoors, the proportions of the natural light spectrum shift significantly into the blue range as soon as the rays of the sun come in at an angle. Wouldn't it be a biological fault if the light receptors of diurnal people and other animals cranked up the internal clock again in the blue hour? In search of an explanation, the scientists used test mice to test exactly how the light stimuli of different colors are converted in the body before they reach the sensors of the internal body clock.

Similar experiments have been carried out before. The importance of the "photosensitive ganglion cells", a third light receptor in the body besides the cones and rods in the retina of the eye, has been known since the beginning of our millennium. These react slowly to differences in the ambient brightness and transmit their signals to the control centers of the internal clock, which then influence the day-night rhythm. The pigment system melanopsin, a protein that reacts to light of different wavelengths and thus activates connected neurons, is central to the ganglion cells. And blue light activates the pigments most strongly - hence the warning why blue light in particular keeps us awake in the evening.

Brown's team also initially confirmed the older findings: blue light tickles melanopsin and the photosensitive ganglion cells particularly strongly. In humans and the experimental mice, however, another system intervenes: the cones stationed in the retina, which contribute different signals at different wavelengths. From the color-dependent input of the cones and the melanopsin ganglion cells, the signal is then generated in downstream connections of the neural network, which then finally transmits in the direction of the internal clock. In their experiments, Brown and colleagues illuminated some genetically modified and normal test animals with differently colored light and noted the reactions in the neural network. It was shown that cones that worked normally and were excited by blue light were actually less active than, for example, those that were activated by yellow light. All in all, this ensured that the effects of cones and ganglion cells cancel each other out - and thus different colors had quite similar effects on the internal clocks.

The decisive factor for the sleep-disturbing effect of light is therefore not so much the color, but rather the brightness, as Brown and Co summarize their findings. For the time being, it remains unclear why this was not recognized in earlier behavioral experiments in which blue light was found to be particularly disturbing. Brown points out that it is not that easy when trying to radiate different colors with really the same intensity. This could well have contributed to the fact that in earlier behavioral experiments blue light was noticed as particularly disturbing: It is not uncommon for experiments to only change the ratio of short to longer wavelengths at the radiation source, which then changes the color impression as desired in the direction of blue or yellow . However, this goes hand in hand with a slight change in intensity, and so the effect of a slightly more intense blue could have been compared with weaker yellow and led to misinterpretations.

Overall, the study confirms that bright light, in particular, makes the body more comfortable to stay awake than to sleep. Technical blue light filters on smartphones and co. Could be helpful or counterproductive depending on the design and ambient lighting, according to Brown - depending on whether they dim the brightness or, on the contrary, even increase it. Even if it may not be perceived in this way psychologically: In fact, warmer, more yellow light could shine us brighter than cooler blue light - and then wake us up rather than make us sleepy.