🔆 Natural light against diabetes

Metabolic diseases, such as diabetes, have reached epidemic proportions. This phenomenon is explained by a sedentary lifestyle coupled with a circadian misalignment—a desynchronization between our internal biological clocks and environmental signals. Furthermore, we spend nearly 90% of our time indoors, with very limited exposure to natural light.

To study the role of the latter on human metabolism, and particularly on blood sugar control, a team from the University of Geneva (UNIGE), the Geneva University Hospitals (HUG), Maastricht University and the German Diabetes Center (DDZ) conducted a controlled study on thirteen volunteers with type 2 diabetes. When exposed to daylight, these individuals showed more stable blood sugar 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 this disease.

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 rhythm is controlled by a central clock located in the brain, which synchronizes those in peripheral organs, for example in the liver or skeletal muscles.

"For several years, we have known that disruption of circadian rhythms plays a major role in the development of metabolic disorders affecting a growing proportion of the Western population," notes Charna Dibner, associate professor at the UNIGE Faculty of Medicine and at HUG, who co-led this work with Joris Hoeks, associate professor at Maastricht University, and Patrick Schrauwen, professor at the DDZ.

In people exposed to natural light, blood glucose levels were in the normal range for longer, with less variability.

"We spend most of our days under artificial lighting, whose light intensity is lower and whose wavelength spectrum is narrower than natural light, which is more effective at synchronizing the biological clock with the environment. Could the lack of natural light be responsible for metabolic diseases such as type 2 diabetes?" adds Joris Hoeks.

Deciphering the body's clocks

The scientists recruited thirteen volunteers aged 65 and over, living with type 2 diabetes. These individuals spent four and a half days in specially designed living spaces at Maastricht University, illuminated either by natural light from large windows or by artificial light. After a break of at least four weeks, they participated in a second session, this time in the other lighting environment.

"This experimental model allows us to examine the same people under both conditions, which limits individual variability in our results," explains Joris Hoeks. "With the exception of the light source, all other lifestyle-related parameters—meals, sleep, physical activity, screen time, etc.—were strictly identical."

And even in the short duration of the experiment, a significant impact was observed: in people exposed to natural light, blood glucose levels were in the normal range for longer, with less variability. "These are two important elements indicating that our diabetic volunteers managed to better control their blood sugar levels," explains Patrick Schrauwen from the DDZ. "Furthermore, their melatonin levels were slightly higher in the evening, and the oxidative metabolism of fats was also improved."

To better understand these positive metabolic changes, the scientists collected blood and muscle samples before, during, and after each lighting regimen. "We analyzed the regulation of molecular clocks in cultured skeletal muscle cells, as well as lipids, metabolites, and gene transcription in the blood. All these results clearly show that the internal clock and metabolism are influenced by natural light. This could explain the improved regulation of blood sugar and the better coordination between the central clock in the brain and the clocks in the organs," explains Charna Dibner.

Results to be confirmed in real-world conditions

This first controlled crossover study, the first of its kind in the world, involved only a small group of elderly people with type 2 diabetes and for only a few days. Nevertheless, it provides the first evidence of the beneficial effect of natural light on metabolic health compared to artificial light. "The next step will be to study the interactions between natural light exposure and metabolic health in real-world conditions, by equipping volunteers with light detectors and glucose measurement tools for several weeks," explains Jan-Frieder Harmsen, lead author of the study, former PhD student in Joris Hoeks' research group and currently a postdoctoral researcher at RWTH Aachen University in Germany. "This study also highlights the often-overlooked impact of building architecture on our health."

This study was conducted with the support of the VELUX Stiftung and the Daylight Academy, the Swiss National Science Foundation, and the Vontobel Foundation.