Natural daylight improves blood sugar control in people with type 2 diabetes

A controlled cross-over study has shown that exposure to natural daylight stabilises blood glucose levels and improves metabolic regulation in older adults with type 2 diabetes, highlighting an overlooked role for light environment in metabolic health

Metabolic diseases have reached epidemic proportions in many societies, driven by sedentary lifestyles and circadian misalignment, defined as a desynchrony between intrinsic biological clocks and external environmental cues. This disruption has intensified as people now spend close to 90 per cent of their time indoors, with limited exposure to natural daylight. Researchers have therefore examined whether daylight itself plays a specific role in human metabolism, with a particular focus on glycaemic control in people with type 2 diabetes.

Scientists from the University of Geneva, the University Hospitals of Geneva, both in Switzerland, Maastricht University in the Netherlands and the German Diabetes Center, Düsseldorf, Germany, have conducted a tightly controlled study involving thirteen volunteers diagnosed with type 2 diabetes. When participants were exposed to natural daylight, they showed more stable blood glucose levels across the day alongside an overall improvement in metabolic indicators. The findings represent the first experimental evidence to demonstrate a direct metabolic benefit of natural light exposure in people with type 2 diabetes.

Human physiology, like that of all living organisms, operates under the influence of circadian rhythms governed by the regular alternation of day and night. A central biological clock in the brain coordinates subsidiary clocks located in peripheral tissues, including the liver and skeletal muscle, to ensure metabolic processes remain aligned with environmental cycles.

“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,” said Dr. Charna Dibner, associate professor at the University of Geneva Faculty of Medicine and at the University Hospitals of Geneva, who co-directed the study with Dr. Joris Hoeks, associate professor at Maastricht University, and Patrick Schrauwen, professor at the German Diabetes Center.

“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 to synchronise the biological clock with the environment. Could the lack of natural light be to blame for metabolic diseases such as type 2 diabetes?” said Hoeks.

The research team recruited thirteen volunteers aged 65 years and above, all living with type 2 diabetes. Each participant spent four and a half days in specially designed residential facilities at Maastricht University. These environments were illuminated either by natural daylight through large windows or by artificial lighting alone. After a washout period of at least four weeks, participants returned to complete a second session under the alternate lighting condition.

“This experimental model allows us to examine the same people under both conditions, which limits individual variability in our results,” said Hoeks.

“Apart from the light source, all other lifestyle parameters – meals, sleep, physical activity and screen exposure – were kept strictly identical.”

Despite the short duration of each exposure period, the metabolic effects were clear. Participants exposed to natural daylight maintained blood glucose levels within the normal range for more hours each day and exhibited reduced glucose variability.

“Two important elements indicate that our volunteers with diabetes managed to control their sugar levels better,” said Dr. Patrick Schrauwen.

“In addition, their melatonin level was slightly higher in the evening, and fat oxidative metabolism also improved.”

To explore the biological mechanisms underlying these changes, the researchers collected blood and skeletal muscle samples before, during and after each lighting condition. Laboratory analyses examined molecular clock regulation in cultured muscle cells alongside lipid profiles, metabolite concentrations and gene transcript patterns in blood samples.

“We analysed the regulation of molecular clocks in skeletal muscle cells together with lipids, metabolites and gene transcripts in the blood,” said Dibner.

“Together, the results clearly show that the internal clock and metabolism are influenced by natural light. This may explain both the improved blood sugar regulation and the better coordination between the central clock in the brain and clocks in peripheral organs.”

The investigators described the work as the first controlled cross-over study of its kind to examine the metabolic effects of natural daylight. Although the cohort size was small and the intervention periods were short, the findings provide robust initial evidence that natural daylight exposure offers metabolic benefits compared with the artificial lighting conditions that dominate modern indoor environments.

“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,” said Dr Jan-Frieder Harmsen, lead author of the study, former doctoral researcher in Hoeks’ group and now a post-doctoral researcher at RWTH Aachen University in Germany.

The authors added that the study underlined the often overlooked influence of building design and lighting architecture on long-term metabolic health, with implications that extend beyond diabetes to broader public health and urban planning considerations.

For further reading please visit: 10.1016/j.cmet.2025.11.006