Lying in a warm bed in the evening watching a film, reading the latest news from friends on your smartphone, or answering the latest emails on your laptop. What is commonplace in many households, ultimately has an effect on sleep and our internal clock. The influence of light plays a significant role here, but what significance does the colour of light have?
Prof. Dr. Manuel Spitschan, Head of the Assistant Professorship of Chronobiology & Health, investigated this question in collaboration with first author Dr. Christine Blume and other colleagues from the University of Basel. Under the title "Effects of calibrated blue-yellow changes in light on the human circadian clock", the scientists investigated the influence of different light colours on the human body. The study was published in the journal "Nature Human Behaviour". The journal has an impact factor of 24.25.
"Vision is a complex process. Light in the evening suppresses melatonin, our body's sleep hormone, and affects our internal clock. So every time we are exposed to light, it synchronizes. We then asked ourselves which light colour has a very specific influence on this," explains Prof. Spitschan and adds: "Basically, there are two light-sensitive types of cells in the human system, rods and cones, which enable vision, reading and colour recognition, as well as the light-sensitive ganglion cells. We wanted to find out what influence these have on the internal clock."
As part of the research project, the research group exposed 16 healthy test subjects to a bluish or yellowish light stimulus in the late evening and a white light stimulus as a control condition. The light stimuli were designed so that they activated the colour-sensitive cones in the retina differently in a controlled manner. However, the stimulation of the light-sensitive ganglion cells was the same in all three conditions. Differences in the effect of the light were therefore directly attributable to the cones' respective stimulation and ultimately the light's color.
"This method of light stimulation allows us to separate the light properties that may play a role in how light affects humans," says Prof. Spitschan. To understand what effect the different light stimuli had on the body, the researchers determined in the sleep laboratory whether the internal clock of the test subjects had changed depending on the colour of the light, how long it took the test subjects to fall asleep and how deep their sleep was at the beginning of the night. They also enquired about their tiredness and tested their ability to react, which decreases with increasing sleepiness.
Neuroscientist Spitschan summarizes the effects of the results as follows: "It was shown that the variation in light colour primarily does not influence the internal clock or sleep, but the brightness does. Light has this adjusting factor and is the most important zeitgeber for us to tell whether it is day or night. And if you see light at night or expose yourself to light at night, it's like a signal to your body: it's daytime, shift your internal clock so that it's right again." This study thus contradicts a study from 2019, which showed that yellow light has a more substantial influence on the internal clock in mice than blue light.
Prof. Spitschan sees the study as an essential step towards putting basic research into practice: "Our findings show that the effect of light on the light-sensitive ganglion cells should probably be considered when planning and designing lighting. The cones and therefore the colour play a very subordinate role. This is an important component for people who work in this area."
The study was published as a registered report in the journal "Nature Human Behaviour." In the two-stage process, the methodology and the project were first described, submitted and finally reviewed. The data was then analyzed and the results presented.
To the publication "Effects of calibrated blue-yellow changes in light on the human circadian clock" in the journal "Nature Human Behaviour"
To the homepage of the Rudolf Mößbauer Assistant Professorship of Chronobiology & Health
To the press release of the University of Basel
Contact:
Prof. Dr. Manuel Spitschan
Rudolf Mößbauer Assistant Professorship of Chronobiology & Health
Georg-Brauchle-Ring 60/62
80992 München
phone: 089 289 24544
e-mail: manuel.spitschan(at)tum.de
Text: Bastian Daneyko/Noëmi Kern (Univ. Basel)
Photos: "Nature Human Behaviour"/private