A controlled crossover study shows that simply working in natural daylight, rather than standard artificial lighting, can stabilize daily glucose fluctuations, enhance fat oxidation, and subtly regulate the body’s metabolic clock in people with type 2 diabetes.
Study: Natural daylight during office hours improves glucose control and whole-body substrate metabolism. Image credit: Piotr Zajda/Shutterstock.com
In a recent study published in Cellular Metabolismresearchers investigated whether office hours in natural daylight, rather than artificial office lighting, might improve health markers in people with type 2 diabetes.
They found that exposure to natural light shifted metabolism toward greater fat oxidation, regulated selected circadian outputs, and altered molecular metabolic signatures. People with more exposure to natural light also showed a modest but statistically significant increase in the amount of time their glucose levels stayed in the normal range.
Why daylight matters for glucose and metabolic health
The human circadian system synchronizes metabolism and physiology with the day-night cycle, with light acting as its most powerful regulator. The central biological clock in the brain coordinates peripheral clocks in organs including the liver, skeletal muscle, and pancreas, influencing glucose metabolism, energy utilization, and insulin sensitivity.
Disruptions of circadian rhythms, which are common in modern indoor-dominant lifestyles, have been strongly linked to metabolic disorders, including type 2 diabetes. People typically spend 80% to 90% of their time indoors, where lighting is lower, spectrally static, and poorly aligned with natural daylight patterns.
Previous studies show that exposure to artificial light can affect glucose and lipid metabolism. However, these studies rarely reflect actual daylight conditions and often focus on short-term or isolated metabolic effects.
Comparison of window daylight with standard artificial office lighting
The researchers aimed to comprehensively assess metabolic, circadian and other physiological responses to natural daylight exposure. They used a randomized crossover trial involving 13 older adults with type 2 diabetes who completed two 4.5-day intervention periods. One period involved exposure to natural daylight while indoors through large windows, and the other involved exposure to constant artificial office lighting intentionally low in melanopic and short-wavelength content.
There was a washout period of four weeks or more between interventions. During each intervention, participants remained continuously in a research facility, followed standardized sleep schedules and meal schedules, and maintained stable medication use.
Exposure to natural daylight occurred during office hours (08:00–17:00), while artificial lighting provided 300 lux at eye level. Exposure to evening light was tightly controlled in both conditions, and blue light-blocking glasses were used when participants left the controlled environment.
Throughout the intervention, continuous glucose monitoring was used to assess glycemic control. Whole-body energy expenditure and substrate oxidation were measured using indirect calorimetry, which included assessments in a breathing chamber and a ventilated hood.
Blood samples were collected over a 24-hour period for metabolic profiling, and a mixed meal tolerance test assessed postprandial metabolism. Skeletal muscle biopsies were obtained to examine clock gene expression and circadian properties in cultured muscle cells. Polyomic analyses, including lipidomics, metabolomics, and single-cell transcriptomics, were performed in an exploratory hypothesis-generating framework to capture systemic molecular responses.
Daylight stabilizes glucose changes and enhances fat oxidation
Exposure to natural daylight did not change mean glucose levels, but resulted in a greater proportion of time within the normal glucose range, indicating improved glycemic stability.
Computational modeling showed that natural light reduced the range of diurnal glucose fluctuations, which was associated with better glucose control. Whole-body energy expenditure was similar between lighting conditions. However, natural daylight consistently shifted metabolism toward higher fat oxidation and lower carbohydrate oxidation during the day and after a mixed meal, reflecting improved metabolic flexibility or the ability to efficiently switch between fuel sources.
Although 24-h plasma glucose, triglyceride, and free fatty acid levels did not differ significantly between conditions, postprandial metabolic dynamics did differ, with natural light promoting a metabolic profile consistent with enhanced lipid utilization. Evening melatonin secretion was higher after exposure to natural daylight, suggesting subtle circadian effects, although melatonin onset time remained unchanged.
At the molecular level, skeletal muscle biopsies showed increased expression of specific clock genes after exposure to natural light. Primary muscle cells cultured from these biopsies exhibited a phase-advanced circadian rhythm, suggesting persistent changes in the properties of the regional clock, as observed ex vivo under controlled laboratory conditions, indicating a possible cellular-level memory of previous light exposure.
Multi-omic analyzes revealed consistent daylight-related patterns in circulating metabolites, lipid classes, and immune cell gene expression, particularly in lipid metabolism pathways. However, most individual molecular features did not remain significant after correction for multiple testing.
These findings indicate that exposure to indoor natural daylight favorably affects glucose regulation, metabolic flexibility, circadian biology, and molecular metabolic signatures in individuals with type 2 diabetes.
Natural light can support diabetes management beyond medication
This study suggests that chronic lack of natural light may be a contributing factor to poor metabolic health in people with type 2 diabetes.
Compared to standard artificial office lighting, exposure to natural light increased the time participants showed glucose readings within normal range and promoted greater fat oxidation, indicating improved metabolic flexibility.
These benefits were accompanied by reduced diurnal glucose fluctuations, higher evening melatonin levels, indicative developments in skeletal muscle circadian phase, and exploratory changes in circulating metabolites, lipids, and immune gene expression associated with insulin sensitivity and lipid metabolism.
A key strength of the study is the randomized crossover design, which features tightly controlled light exposure, meals, and activity. However, the small sample size, short intervention duration, larger study population, seasonal restriction, and reliance on subjective sleep measures limit generalizability and warrant cautious interpretation of causality.
Overall, the findings highlight natural daylight as a potentially modifiable environmental factor that may support metabolic control in type 2 diabetes and warrant larger, longer, and more naturalistic real-world studies, particularly in working-age populations and in real-world office settings.
Journal Reference:
Harmsen, J., Habets, I., Biancolin, AD, Lesniewska, A., Phillips, NE, Metz, L., Sanchez-Avila, J., Kotte, M., Timmermans, M., Hashim, D. de Kam, SS, Schaart, G., Jörgenni, AE, Doligkeit, D., van de Weijer, T., Buitinga, M., Haans, F., De Lorenzo, R., Pallubinsky, H., Gordijn, MCM, Collet, T., Kramer, A., Schrauwen, P., Dibner, C., Hoeks, J. (202). Natural daylight during office hours improves glucose control and whole body substrate metabolism. Cellular Metabolism 38(1). DOI: 10.1016/j.cmet.2025.11.006.
