Blue light filters in smartphones and computer displays have become popular in recent times. Researchers at the University of Salzburg have now investigated whether they really do contribute to better sleep. Their preliminary result: yes, partially, they write in the scientific journal Clock Sleep. Short-wave light not only influences circadian rhythms and evening sleepiness, but also sleep physiology and wakefulness in the morning. Using a blue light filter in the evening reduces these negative effects.
For their pilot study, psychologists Sarah Schmid and Christopher Hohn sent 14 young men to a sleep laboratory for three nights. Beforehand, they had to read the same texts in the same font size on a smartphone with and without blue light filters as well as on paper for 90 minutes, they describe in their paper. The titles were "The Kangaroo Chronicles: Views of a noisy marsupial", "555 popular misconceptions: Why Angela Merkel is actually a West German, eggs don’t have to be discouraged and peanuts are not nuts" and "13 against the summer hole: 13 authors – 13 stories – 13 x Lesespab".
Throughout the night, their bodily functions were continuously monitored. Polysomnography was used to measure brain waves, eye movements and muscle activity, as well as body temperature regulation and changes in the hormones cortisol and melatonin.
The natural increase in cortisol the next morning after reading on a cell phone at night without a blue light filter was reduced compared to reading with a filter, which means that the subjects woke up more slowly. Overall, this suggests that cortisol levels were also elevated during the night after reading without the blue light filter. Reduced deep sleep in the first sleep quarter was also seen after reading without blue light filters. Smartphone readers’ sleep was more fragmented, subjects woke up more often during the night.
After reading a book, evening melatonin levels were higher than after reading on a smartphone, whether with or without a blue light filter. With vasodilation, the dilation of blood vessels, yet another relaxation parameter was at its grossest after reading in a book.
However, the findings on attention during reading on the smartphone in the evening do not seem to match: subjectively, sleepiness was reduced during reading on the smartphone, but objectively, no increased attention performance, i.e. no better reaction times, was observed.
"The pineal gland in the brain, which has a direct connection to our eyes, produces the sleep hormone melatonin. When it gets dark, melatonin production increases, we get mude", explains psychologist and sleep researcher Kerstin Hodlmoser from the Center for Cognitive Neuroscience at the University of Salzburg. When bright light enters the eyes, the production of melatonin is inhibited, the sleep-wake rhythm shifts. This effect is strongest with blue light around 450 nanometers, while long-wave red light has no effect on melatonin production.