Spending time in natural light during regular work hours tends to better maintain blood sugar levels and burn more body fat in people with type 2 diabetes. This metabolic change provides evidence that environmental lighting plays an important role in the management of chronic health conditions. The results of this study were recently published in the journal cell metabolism.
“We are working in the field of type 2 diabetes, especially the metabolic aspects of type 2 diabetes,” said Patrick Schrauwen, a researcher at the Institute of Clinical Diabetes at the German Diabetes Center. The human body relies on circadian rhythms, which are internal body clocks that regulate sleep, digestion, and hormone release. Light is the primary signal that synchronizes this internal clock with the outside world.
“Sunlight affects this clock, making our metabolism rhythmic and adapted to life on Earth with a 24-hour day/night cycle,” Schrauwen explained. The research team previously found that people at risk for type 2 diabetes often exhibit disrupted metabolic rhythms.
“We don’t know why, but we do know that we spend much of our working time indoors without exposure to sunlight,” Schrauwen said. Inspired by this, scientists conducted a controlled proof-of-concept study to see if sunlight could directly alter human metabolism.
To test this, researchers conducted a randomized crossover trial of 13 volunteers with well-controlled type 2 diabetes. The group consisted of eight women and five men with an average age of 70 years. Each participant completed two separate testing periods of four and a half weeks each, with a four-week rest period in between.
During the intervention, volunteers stayed in a controlled research facility. For a time, they spent their daytime hours sitting at desks facing wide windows that let in dynamic natural light. The rest of the time, they sat at desks in the same room, separated by a light-blocking barrier, and exposed only to standard office artificial lighting set at approximately 300 lux.
The scientists closely controlled the participants’ daily routines to isolate the effects of lighting. Everyone ate standardized meals at precise times to ensure that daily data did not change due to digestion. They also similarly performed light physical activity, such as climbing stairs, for 30 minutes after each meal.
If participants had to leave the room under artificial light, they wore special orange-tinted glasses. These glasses completely block blue light, the type of light that most strongly affects your circadian clock. This ensures that participants are not accidentally exposed to outdoor natural light during artificial light trials.
To measure blood sugar levels, volunteers wore continuous blood sugar monitors. This is a small sensor attached to your arm that tracks your blood sugar levels around the clock. The researchers also measured oxygen consumption and carbon dioxide production using special breathing masks and sealed breathing chambers. This technology allows scientists to calculate how many calories your body is burning and whether it uses carbohydrates or fat for energy.
Exposure to natural light significantly improved metabolism. Although their overall average blood sugar levels did not change, participants spent more time in a normal, healthy blood sugar range when exposed to natural light. Specifically, it stayed in this healthy range about 51% of the time under natural light, compared to 43% under artificial light.
Continuous blood glucose monitoring also showed that natural light produced smaller rises and falls in blood sugar levels over a 24-hour period. As these blood sugar levels stabilized, researchers observed changes in the way the body uses energy. When exposed to natural light during the day, participants burned less carbohydrates and oxidized more fat.
Through extensive blood analysis, the scientists looked at a wide range of circulating metabolites, which are small molecules produced during digestion and energy use. Under natural light, participants had higher levels of cholic acid and glutamate. Ceramides, a type of fat that is often increased in people with type 2 diabetes, tended to decrease under natural light.
When examining the muscle tissue taken at the end of the study, the scientists found that certain genes that control the circadian clock were more active. The researchers induced these cells to grow into mature muscle fibers in a Petri dish. By inserting tracking proteins, we observed that the daily rhythmic patterns of these cells changed after exposure to natural light.
“We expected to find an effect of sunlight, but we were surprised at how widespread it was, given that this was just a short-term study,” Schrauwen said. “In addition to glucose, there may also be effects on some metabolites in the blood and on muscle. We think the effects could be even stronger if long-term effects are investigated.”
“We live in a world where our natural 24-hour rhythms can easily be disrupted,” Schrauwen says. “We eat food all day long, work late, and are exposed to artificial light right before bed,” he said, explaining that while these disruptions can increase the risk of type 2 diabetes, exposure to sunlight may reset the body’s internal clock and lower the risk.
Although these findings are promising, the study included a small sample of only 13 older adults, which limits the applicability of the results to the general population. Also, the experiment was conducted strictly during the summer, so the effects of natural light may be different on dark winter days. Additionally, sleep quality was only measured by subjective questionnaires, rather than objective EEG monitoring.
Schrauwen pointed out that this was a short-term intervention study comparing just five days of sunlight to artificial light. “Longer-term studies are needed to investigate the true impact on our metabolic health,” he added.
“We aim to further understand how disruption of 24-hour metabolic rhythms leads to diabetes and how proper 24-hour rhythms can be restored,” Schrauwen said. In addition to light exposure, scientists plan to focus on the effects of how people eat and exercise. Exploring how these lifestyle factors interact with our body clocks may ultimately lead to new guidelines that prioritize natural light at home and at work.
The study, “Natural light during work hours improves glycemic control and whole-body substrate metabolism,” was authored by Jan Frieder Harmsen, Ivo Habetz, Andrew D. Biancolin, Agata Lesniewska, Nicholas E. Phillips, Loic Metz, Juan Sánchez Ávila, Marit Cotte, Merel Timmermans, Jancel Hashim, Soraya S. de Cam, and Gert. Schaardt, Johanna A. Jorgensen, Anne Gemminck, Esther Moonen-Kornips, Daniel Drigkeit, Tineke van de Weijer, Mike Buitinga, Florian Hearns, Rebecca de Lorenzo, Hanna Palvinski, Marieke CM Godin, Tinghai Colette, Achim Kramer, Patrick Schrauwen, Charna Dibner and Joris Fuchs.
