Natural daylight during office hours improves glucose stability in type 2 diabetes

Natural daylight during office hours improves glucose stability in type 2 diabetes

A controlled crossover study shows that in people with type 2 diabetes, working in natural daylight alone instead of traditional artificial lighting can stabilize daily glucose fluctuations, increase fat oxidation, and subtly tune the body's metabolic clock.

Study: Natural daylight during office hours improves glucose control and substrate metabolism throughout the body. Photo credit: Piotr Zajda/Shutterstock.com

In a recent study published inCell metabolismResearchers examined whether spending office hours in natural daylight instead of artificial office lighting can improve health indicators in people with type 2 diabetes.

They found that exposure to natural light shifted metabolism toward greater fat oxidation, modulated select circadian outputs, and altered molecular metabolic signatures. People who were more exposed to natural light also experienced a slight but statistically significant increase in the time their glucose levels remained within normal ranges.

Why daylight is important for glucose and metabolic health

The human circadian system synchronizes metabolism and physiology with the day-night cycle, with light acting as the most powerful regulator. The central biological clock in the brain coordinates peripheral clocks in organs including liver, skeletal muscle, and pancreas, influencing glucose metabolism, energy expenditure, and insulin sensitivity.

Circadian rhythm disruptions, common in modern indoor living-dominated lifestyles, are closely linked to metabolic disorders, including type 2 diabetes. People typically spend 80 to 90% of their time indoors, where lighting is dim, spectrally static, and poorly matched to natural daylight patterns.

Previous studies suggest that exposure to artificial light may affect glucose and lipid metabolism; However, these studies rarely reflect real-world daylight conditions and often focus on short-term or isolated metabolic outcomes.

Comparison of window daylight with conventional artificial office lighting

The researchers' goal was 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. In one period, employees were exposed to natural daylight indoors through large windows; in the other, they were exposed to constant artificial office lighting that was deliberately limited to melanopic and short-wave components.

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 times, and maintained consistent medication use.

During office hours (8:00 a.m.–5:00 p.m.), exposure was with natural daylight, while artificial lighting at eye level provided 300 lux. In both conditions, evening light exposure was strictly controlled and glasses that blocked blue light were used when leaving the controlled environment.

Continuous glucose monitoring was performed throughout the intervention to assess glycemic control. Whole body energy expenditure and substrate oxidation were measured using indirect calorimetry, which included assessments in a ventilation chamber and a ventilated hood.

To create a metabolic profile, blood samples were collected over a 24-hour period and a mixed meal tolerance test examined postprandial metabolism. Skeletal muscle biopsies were performed to examine clock gene expression and circadian properties in cultured muscle cells. Multiomic analyses, including lipidomics, metabolomics, and monocyte transcriptomics, were performed in an exploratory, hypothesis-generating framework to capture systemic molecular responses.

Daylight stabilizes glucose fluctuations and promotes fat oxidation

Exposure to natural daylight did not alter average glucose levels but resulted in a greater proportion of time spent within the normal glucose range, indicating improved glycemic stability.

Computer models showed that natural light reduced the amplitude of diurnal glucose fluctuations, which was associated with better glucose control. Whole body energy expenditure was similar under different lighting conditions. However, natural daylight continuously shifted metabolism toward greater fat oxidation and lower carbohydrate oxidation throughout the day and after a mixed meal, reflecting improved metabolic flexibility, or the ability to switch efficiently between energy sources.

Although 24-hour levels of plasma glucose, triglycerides, and free fatty acids did not differ significantly between conditions, postprandial metabolic dynamics differed, with natural light promoting a metabolic profile consistent with improved lipid utilization. Evening melatonin secretion was higher after exposure to natural daylight, suggesting subtle circadian effects, although the timing of melatonin onset remained unchanged.

At the molecular level, biopsies of skeletal muscles showed increased expression of specific clock genes after natural light exposure. Primary muscle cells cultured from these biopsies showed a phase-shifted circadian rhythm, suggesting persistent changes in peripheral clock properties as observed ex vivo under controlled laboratory conditions, suggesting possible memory at the cellular level of previous light exposure.

Multi-omic analyzes revealed consistent daylight-associated patterns in circulating metabolites, lipid classes, and immune cell gene expression, particularly involving lipid metabolic pathways. However, most individual molecular features remained nonsignificant after correction for multiple tests.

These results demonstrate that exposure to indoor natural daylight positively influences 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 factor contributing to poorer metabolic health in people with type 2 diabetes.

Compared to traditional artificial office lighting, exposure to natural light extended the time participants' glucose levels were within normal ranges and promoted greater fat oxidation, indicating improved metabolic flexibility.

These benefits were accompanied by lower diurnal glucose fluctuations, higher evening melatonin levels, evidence of advances in the circadian phase of skeletal muscle, and experimental changes in circulating metabolites, lipids, and immune cell gene expression related to insulin sensitivity and lipid metabolism.

A key strength of the study is its randomized crossover design, which includes tightly controlled light exposure, meals and activity. However, the small sample size, short intervention duration, older study population, seasonal limitation, and reliance on subjective sleep measurements limit generalizability and require cautious interpretation of causality.

Overall, the results highlight that natural daylight is a potentially modifiable environmental factor that may support metabolic control in type 2 diabetes and warrants longer, larger, and more naturalistic real-world studies, particularly in working-age populations and real-world office settings.

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