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Spending time in natural daylight during normal working hours tends to help people with type 2 diabetes maintain better blood sugar levels and burn more body fat. This shift in metabolism provides evidence that environmental lighting plays a significant role in managing chronic health conditions. The findings were recently published in the journal Cell Metabolism.
“We are working in the field of type 2 diabetes, and specifically metabolic aspects of type 2 diabetes,” said Patrick Schrauwen, a researcher at the Institute for Clinical Diabetology at the German Diabetes Center. The human body relies on a circadian rhythm, which is an internal twenty-four-hour clock that regulates sleep, digestion, and hormone release. Light is the primary signal that synchronizes this internal clock with the outside world.
“Daylight influences this clock, and makes that our metabolism is rhythmic, in such a way that it is adapted to live on earth, with its 24-hour day and night cycle,” Schrauwen explained. The research team previously found that people at risk for type 2 diabetes often exhibit a disturbed metabolic rhythm.
“The reason for this is not known, but we do know that we spend a lot of our working time inside, without exposure to daylight,” Schrauwen noted. This inspired the scientists to conduct a controlled proof-of-concept study to see if daylight could directly alter human metabolism.
To test this, the researchers conducted a randomized crossover trial involving thirteen volunteers with well-controlled type 2 diabetes. The group consisted of eight women and five men with an average age of seventy years. Each participant completed two distinct testing periods lasting four and a half days each, with a four-week resting period in between.
During the interventions, the volunteers stayed in a controlled research facility. For one period, they spent their daytime hours sitting at a desk facing a wide window that provided dynamic natural daylight. For the other period, they sat at a desk in the same room but were separated by a lightproof barrier and exposed only to standard artificial office lighting set to about three hundred lux.
The scientists maintained strict control over the participants’ routines to isolate the effect of the lighting. Everyone ate standardized meals at exact times to ensure digestion did not alter the daily data. They also performed identical bouts of light physical activity, such as stepping on stairs, for thirty minutes after each meal.
If participants needed to leave the room during the artificial light condition, they wore special orange-tinted glasses. These glasses completely block out blue light, which is the type of light that most strongly affects the circadian clock. This ensured the participants did not accidentally receive a dose of natural outdoor light during their artificial light trial.
To measure blood sugar, the volunteers wore continuous glucose monitors, which are small sensors attached to the arm that track glucose levels around the clock. The researchers also used specialized breathing masks and a sealed respiration chamber to measure oxygen consumption and carbon dioxide production. This technique allows scientists to calculate how many calories the body is burning and whether it is using carbohydrates or fats for energy.
Exposure to natural daylight yielded noticeable improvements in metabolism. While average overall blood sugar levels did not change, participants spent more time in a normal, healthy blood sugar range when exposed to natural light. Specifically, they stayed in this healthy range for about fifty-one percent of the time under natural light, compared to forty-three percent under artificial light.
The continuous glucose monitors also showed that natural light led to smaller spikes and dips in blood sugar over a twenty-four-hour period. Along with these more stable blood sugar levels, the researchers observed a shift in how the body used energy. During the daytime hours, participants burned fewer carbohydrates and oxidized more fat when they were exposed to natural daylight.
Through extensive blood analysis, the scientists looked at a wide array of circulating metabolites, which are small molecules produced during digestion and energy use. Under natural light, participants had higher levels of cholic acid and glutamic acid. Ceramides, a type of fat often elevated in people with type 2 diabetes, showed a tendency to decrease under natural daylight.
When examining muscle tissue collected at the end of the study, the scientists found higher activity in specific genes that control the circadian clock. The researchers coaxed these cells to grow into mature muscle fibers in a laboratory dish. By inserting a tracking protein, they observed that the daily rhythmic patterns of these cells shifted after the natural light exposure.
“Even though we hoped to find effects of daylight, given this was only a short study, we were surprised that the effects of daylight were broad,” Schrauwen said. “Not only on glucose, but also on several metabolites in the blood, and also effects on the muscle. We believe that effects may be even stronger if the longer term effects would be investigated.”
“We live in a world where our natural 24-hour rhythm gets easily disturbed,” Schrauwen said. “We have food all day through, we work late, and we are exposed to artificial light until just before going to sleep.” He explained that these disruptions might increase the risk of type 2 diabetes, while daylight exposure could help reset our biological clock and lower that risk.
While these findings are promising, the study involves a small sample of only thirteen older adults, which limits how broadly the results can be applied to the general population. The experiment also took place strictly during the summer months, meaning the effects of natural light might be different during the darker days of winter. Additionally, sleep quality was only measured through subjective questionnaires rather than objective brainwave monitoring.
Schrauwen pointed out that this was a short intervention study, comparing only five days of daylight versus artificial light. “Longer studies are needed to investigate the true impact on our metabolic health,” he added.
“We aim to further understand how disturbances in our 24-hour rhythmicity of metabolism can lead to diabetes, and how we can restore a proper 24-hour rhythmicity,” Schrauwen noted. In addition to light exposure, the scientists plan to focus on the impact of when people eat and exercise. Exploring how these lifestyle factors interact with the body’s internal clock could eventually lead to new guidelines that prioritize natural daylight in homes and workplaces.
The study, “Natural daylight during office hours improves glucose control and whole-body substrate metabolism,” was authored by Jan-Frieder Harmsen, Ivo Habets, Andrew D. Biancolin, Agata Lesniewska, Nicholas E. Phillips, Loic Metz, Juan Sanchez-Avila, Marit Kotte, Merel Timmermans, Dzhansel Hashim, Soraya S. de Kam, Gert Schaart, Johanna A. Jörgensen, Anne Gemmink, Esther Moonen-Kornips, Daniel Doligkeit, Tineke van de Weijer, Mijke Buitinga, Florian Haans, Rebecca De Lorenzo, Hannah Pallubinsky, Marijke C.M. Gordijn, Tinh-Hai Collet, Achim Kramer, Patrick Schrauwen, Charna Dibner, and Joris Hoeks.
