Doesn't blue light keep us awake?
Light in the evening disturbs our sleep. Blue light in particular is suspected of giving us restless nights – which is why many smartphones already offer the option of using warmer tones in the evening hours. But these filters, according to researchers led by Timothy Brown from the University of Manchester, may not be realistic: According to their experiments, colder, blue light could be even less disruptive than yellow tones.
The scientists used laboratory mice to test how the body processes light stimuli of different wavelengths. Similar experiments have been done before. Among other things, researchers came across the photosensitive ganglion cells, a third light receptor in addition to the cones and rods in the retina of the eye. These ganglion cells react to differences in ambient brightness and transmit their signals to the control centers of our internal clock, which then control the day-night rhythm. Central to this is the protein melanopsin, which reacts to light of different wavelengths and thus activates downstream neurons. It is most active in blue light – which is why we assume that this keeps us awake longer in the evening. But as Brown's team has now observed, another system appears to be intervening in humans and test mice: the cones stationed in the retina, which each contribute different signals at different wavelengths. When the researchers irradiated their test animals differently, cones stimulated by blue light worked less actively than those that react to yellow light, for example. Overall, the effects of cones and ganglion cells canceled out - and all colors of light had fairly similar effects on internal clocks.
The decisive factor for the sleep-disturbing effect of light is therefore less the color than the brightness, Brown and his colleagues summarize their findings. For the time being, it remains unclear why blue light was found to be disruptive to sleep in many other studies. For Brown, this could have something to do with the fact that it's not easy to irradiate different colors with the same intensity. Not infrequently, experiments only change the ratio of short to longer wavelengths at the radiation source, which then changes the color impression in the desired direction. However, this goes hand in hand with a certain change in intensity, and for example in the past the effect of more intense blue could have been compared to that of weaker yellow.