Natural Lighting During Office Hours Improves Blood Glucose Control and Shifts Metabolism
Researchers compared natural daylight to artificial lighting and found real metabolic differences in people with type 2 diabetes.
We spend up to 90% of our time indoors, bathed in artificial light. A new study out of Maastricht University has found that simply swapping that artificial office lighting for natural daylight through a window improved blood sugar control and shifted the body to burn more fat in people with type 2 diabetes. The findings were published in Cell Metabolism in January 2026.
Aim
The researchers wanted to know whether natural daylight during typical office hours, as opposed to standard artificial lighting, would improve glucose control and whole-body metabolism in people with type 2 diabetes (T2D). The study also aimed to understand why, by looking at changes in skeletal muscle clock genes, circulating metabolites, lipids, and immune cell gene expression.
Methods
Thirteen adults with T2D (8 women, 5 men; average age 70 years; average BMI 30) took part in a randomised crossover trial. Each person completed two 4.5-day stints in a research facility, one under natural daylight through wide windows and one under constant artificial lighting set to around 300 lux. The two periods were separated by at least four weeks
Office hours ran from 8 am to 5 pm. Meals, physical activity, sleep schedules, and evening light exposure were standardised and kept identical across both conditions. Participants wore blue-light-blocking glasses whenever they left their assigned room, ensuring that those in the artificial light condition were never exposed to the short wavelengths of natural daylight the human circadian system is most sensitive to.
Continuous glucose monitoring (CGM) ran across the entire 4.5 days. On day 4, researchers used indirect calorimetry to measure how the body burned fuel. Skeletal muscle biopsies, blood samples, and saliva were also collected for deeper molecular analysis.
Results
Glucose control improved under natural daylight. While average glucose levels across the 4.5 days were not statistically different between the two conditions, participants spent significantly more time in the normal glucose range (4.4 to 7.2 mmol/L) under natural light compared to artificial light (51% vs. 43%). The daily swings in baseline glucose levels were also smaller under natural daylight, and those smaller swings were directly linked to more time in the normal range.
The body burned more fat. Under natural daylight, participants showed lower carbohydrate oxidation and higher fat oxidation throughout the waking day, particularly around midday. This pattern held up during a standardised mixed meal test the following morning, with free fatty acid levels also trending higher under natural light.
The muscle clock shifted. Muscle biopsies showed higher expression of the clock genes Per1 and Cry1 after natural light exposure. When muscle cells grown from those biopsies were tested in a lab, their internal circadian rhythm was phase-advanced by about 45 minutes, a sign that the muscle’s internal clock had been genuinely reset by the light environment rather than just temporarily altered.
Melatonin rose higher in the evening. While the timing of melatonin onset did not differ between conditions, melatonin levels in the two hours before bed were meaningfully higher under natural light. This suggests the body’s sleep-wake signal was stronger after a day in natural daylight.
Multi-omic signatures shifted, too. Blood metabolite and lipid profiles also differed between conditions. Levels of ceramides and cholesterol esters, both elevated in T2D and associated with poorer metabolic health, trended lower under natural light. Lipid species linked to better insulin sensitivity were higher. These molecular signals are broadly consistent with the improvements in glucose control and fat burning observed.
Blood pressure, heart rate, core body temperature, physical activity, sleep quality, and mood scores did not differ between conditions, ruling out these factors as drivers of the metabolic differences.
Takeaways
The sample was small. Only 13 participants completed the trial, all older adults (average age 70), so caution is warranted before generalising these findings to younger or broader populations. The researchers note that longer trials are needed to confirm whether these effects hold over weeks or months.
Getting natural daylight through a window during work hours improved time in the normal blood sugar range in people with T2D, without any changes to diet, medication, or exercise
The body shifted toward burning more fat during the day under natural light conditions
The skeletal muscle’s internal clock was measurably reset by daylight, pointing to a plausible biological mechanism
Evening melatonin levels were higher after daytime natural light exposure, which may benefit sleep quality
Artificial office lighting set to a typical 300 lux produced measurably worse metabolic outcomes than natural daylight in this group
The built environment, specifically whether your workplace has windows, may matter more for metabolic health than previously recognised
Reference
Harmsen, J-F., Habets, I., Biancolin, A.D., Lesniewska, A., Phillips, N.E., Metz, L., Sanchez-Avila, J., Kotte, M., Timmermans, M., Hashim, D., de Kam, S.S., Schaart, G., Jörgensen, J.A., Gemmink, A., Moonen-Kornips, E., Doligkeit, D., van de Weijer, T., Buitinga, M., Haans, F., De Lorenzo, R., Pallubinsky, H., Gordijn, M.C.M., Collet, T-H., Kramer, A., Schrauwen, P., Dibner, C., & Hoeks, J. (2026). Natural daylight during office hours improves glucose control and whole-body substrate metabolism. Cell Metabolism, 38, 65–81. https://doi.org/10.1016/j.cmet.2025.11.006
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