Metabolic diseases have reached epidemic proportions in our society, driven by a sedentary lifestyle coupled with circadian misalignment - a desynchrony between our intrinsic biological clocks and environmental signals. Furthermore, we spend almost 90% of our time indoors, with a very limited exposure to natural daylight. To investigate the specific role of daylight in human metabolism, particularly in glycaemic control, researchers from the University of Geneva (UNIGE), the University Hospitals of Geneva (HUG), Maastricht University, and the German Diabetes Center (DDZ) conducted a controlled study with thirteen volunteers with type 2 diabetes. When exposed to natural light, participants exhibited more stable blood glucose levels and an overall improvement in their metabolic profile. These results, published in the journal Cell Metabolism, provide the first evidence of the beneficial impact of natural light on people with type 2 diabetes.
As in all living beings, human physiological processes are subject to the influence of the circadian rhythm governed by the alternation of day and night. This is controlled by a central clock in the brain, which synchronises the clocks in peripheral organs such as the liver and skeletal muscles.
"It has been known for several years that the disruption of circadian rhythms plays a major role in the development of metabolic disorders that affect an increasing proportion of the Western population," notes Charna Dibner, associate professor at the UNIGE Faculty of Medicine and at HUG, who co-directed this work with Joris Hoeks, associate professor at Maastricht University, and Patrick Schrauwen, professor at the DDZ.
"We largely spend our days under artificial lighting, which has a lower light intensity and a narrower wavelength spectrum than natural light. Natural light is also more effective in synchronizing the biological clock with the environment. Could the lack of natural light be to blame for metabolic diseases such as type 2 diabetes?" adds Joris Hoeks.
A look into body clocks
The research team recruited 13 volunteers aged 65 and over, all with type 2 diabetes. They spent 4.5 days in specially designed living spaces at Maastricht University, lit either with natural light through large windows or with artificial light. After a break of at least four weeks, they returned for a second session, this time in the other light environment.
"This experimental model allows us to examine the same people under both conditions, which limits the individual variability in our results," explains Joris Hoeks. "Apart from the light source, all the other lifestyle parameters - meals, sleep, physical activity, screen time, etc. - were kept strictly identical."
Surprisingly, even over the short duration of the experiment, a significant impact was noted: in people exposed to natural light, blood glucose levels were in the normal range for more hours per day, with less variability. "Two important elements that indicate that our volunteers with diabetes managed to control their sugar levels better," says Patrick Schrauwen from DDZ. "In addition, their melatonin level was a little higher in the evening, and fat oxidative metabolism was also improved."
To better understand the observed positive changes in the body's metabolism, the scientists took blood and muscle samples from the volunteers before, during, and after each light treatment. "We analysed the regulation of molecular clocks in cultured skeletal muscle cells together with lipids, metabolites, and gene transcripts in the blood. Together, the results clearly show that the internal clock and metabolism are influenced by natural light. This could be the reason for the improved blood sugar regulation and the improved coordination between the central clock in the brain and the clocks in the organs," explains Charna Dibner.
Results should be confirmed in real-life conditions
This study - a world first controlled cross-over study - only involved a small cohort of older people with type 2 diabetes over a short period of time. Nevertheless, it is the first evidence of the beneficial effect of natural daylight on metabolic health as compared to artificial light to which we tend to be exposed most of the time. "The next step will be to study the interactions between exposure to natural light and metabolic health in real-life conditions, by equipping volunteers with light detectors and glucose measurement tools for several weeks," say Jan-Frieder Harmsen, lead author of the study, former PhD fellow in Hoeks' research group and currently a post-doctoral researcher at RWTH Aachen University, Germany. "This study also highlights the often-overlooked impact of building architecture on our health.”
This study was carried out with the support of the VELUX Stiftung and Daylight Academy, the Swiss National Science Foundation and the Vontobel Foundation.