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 synchronizes the “clocks” in organs such as the liver and skeletal muscles. “We have known for some 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,” explains Professor Charna Dibner from the UNIGE Faculty of Medicine and the HUG. She led the study together with Professor Joris Hoeks of Maastricht University and Professor Patrick Schrauwen from 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 more effective in synchronizing the internal biological clock with the environment. We wanted to know whether the lack of natural light could be to blame for metabolic diseases such as type 2 diabetes,” adds Hoeks.
More Stable Blood Sugar Values in Natural Light
The research team recruited 13 volunteers with type 2 diabetes aged 65 or older. 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 Hoeks. “Apart from the light source, all the other lifestyle parameters – meals, sleep, physical activity, screen time, etc. – were kept strictly identical.”
Despite the short duration of the experiment, the team observed significant differences: In people exposed to natural light, blood sugar values 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 blood sugar levels better,” explains Patrick Schrauwen. He is a scientist in the Energy Metabolism Research Group at the DDZ, which is headed by Professor Michael Roden, Scientific Director and Spokesman of the Board of the DDZ as well as Director of the Clinic for Endocrinology and Diabetology at the University Hospital Düsseldorf (UKD). “In addition, their melatonin level was a little higher in the evening, and fat oxidative metabolism was also improved,” adds Schrauwen.
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 analyzed 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.
The Results Should Be Confirmed in Real-Life Conditions
This study—a world first controlled cross-over study (one person, multiple treatment conditions)—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 metabolism as compared to artificial light, which people are exposed to 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,” says 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. “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 the Daylight Academy, the Swiss National Science Foundation, and the Vontobel Foundation.
Original publication:
Harmsen, J. F., Habets, I., Biancolin, A. et al. Natural daylight during office hours improves glucose control and whole-body substrate metabolism. Cell Metabolism. Doi: https://doi.org/10.1016/j.cmet.2025.11.006
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The German Diabetes Center (DDZ) serves as the German reference center for diabetes. Its objective is to contribute to the improvement of prevention, early detection, diagnosis and treatment of diabetes mellitus. At the same time, the research center aims at improving the epidemiological data situation in Germany. The DDZ coordinates the multicenter German Diabetes Study and is a point of contact for all players in the health sector. In addition, it prepares scientific information on diabetes mellitus and makes it available to the public. The DDZ is part of the Leibniz Association (Wissenschaftsgemeinschaft Gottfried Wilhelm Leibniz, WGL) and is a partner of the German Center for Diabetes Research (DZD e.V.). www.ddz.de/en
The German Center for Diabetes Research (DZD) is a national association that brings together experts in the field of diabetes research and combines basic research, translational research, epidemiology and clinical applications. The aim is to develop novel strategies for personalized prevention and treatment of diabetes. Members are Helmholtz Munich – German Research Center for Environmental Health, the German Diabetes Center in Düsseldorf, the German Institute of Human Nutrition in Potsdam-Rehbrücke, the Paul Langerhans Institute Dresden of Helmholtz Munich at the University Medical Center Carl Gustav Carus of the TU Dresden and the Institute for Diabetes Research and Metabolic Diseases of Helmholtz Munich at the Eberhard-Karls-University of Tuebingen together with associated partners at the Universities in Heidelberg, Cologne, Leipzig, Lübeck and Munich. www.dzd-ev.de/en
