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

By Priyanjana Pramanik, MSc.Reviewed by Lauren HardakerJan 8 2026

A controlled crossover study shows that simply working in natural daylight, rather than standard artificial lighting, can stabilize daily glucose swings, boost fat oxidation, and subtly tune 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 Cell Metabolism, researchers investigated whether spending office hours in natural daylight, rather than 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 with greater natural light exposure also experienced a modest but statistically significant increase in the time when their glucose levels remained in the normal range.

Why daylight matters for glucose and metabolic health

The human circadian system synchronizes metabolism and physiology to 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 use, and insulin sensitivity.

Disruptions to circadian rhythms, which are common in modern lifestyles dominated by indoor living, have been strongly linked to metabolic disorders, including type 2 diabetes. Individuals typically spend 80 % to 90 % of their time indoors, where lighting is dimmer, spectrally static, and poorly aligned with natural daylight patterns.

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

Comparing window daylight versus standard artificial office lighting

Researchers aimed to comprehensively assess metabolic, circadian, and other physiological responses to natural daylight exposure. They used a randomized crossover trial that included 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 stayed continuously in a research facility, followed standardized sleep schedules and meal timings, and maintained consistent medication use.

Natural daylight exposure occurred during office hours (08:00–17:00), while artificial lighting provided 300 lux at eye level. Evening light exposure was tightly controlled in both conditions, and glasses that blocked blue light were used when participants left the controlled environment.

Continuous glucose monitoring was used throughout the intervention to assess glycemic control. Whole-body energy expenditure and substrate oxidation were measured using indirect calorimetry, which included assessments in a respiration chamber and 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. Multi-omic analyses, including lipidomics, metabolomics, and monocyte transcriptomics, were performed in an exploratory, hypothesis-generating framework to capture systemic molecular responses.

Daylight stabilizes glucose swings and boosts 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.

Computational modeling showed that natural light reduced the amplitude of daily 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 greater fat oxidation and lower carbohydrate oxidation throughout the day and following a mixed meal, reflecting improved metabolic flexibility, or the ability to switch efficiently between fuel 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 enhanced lipid utilization. Evening melatonin secretion was higher following natural daylight exposure, suggesting subtle circadian effects, although the timing of melatonin onset remained unchanged.

At the molecular level, biopsies of skeletal muscle showed increased expression of specific clock genes following natural light exposure. Primary muscle cells cultured from these biopsies exhibited a phase-advanced circadian rhythm, suggestive of persistent alterations in peripheral clock properties, as observed ex vivo under controlled laboratory conditions, indicating a potential cellular-level memory of prior light exposure.

Multi-omic analyses revealed consistent daylight-associated patterns in circulating metabolites, lipid classes, and immune-cell gene expression, particularly involving lipid metabolism pathways. However, most individual molecular features did not remain significant after correction for multiple testing.

These findings demonstrate that exposure to indoor natural daylight favorably influences glucose regulation, metabolic flexibility, circadian biology, and molecular metabolic signatures in individuals with type 2 diabetes.

Natural light may support diabetes management beyond medication

This study suggests that chronic lack of natural light may be one contributing factor to poorer metabolic health in people with type 2 diabetes.

Compared with standard artificial office lighting, exposure to natural light increased the time that participants showed glucose readings within a normal range and promoted greater fat oxidation, indicating improved metabolic flexibility.

These benefits were accompanied by reduced daily glucose fluctuations, higher evening melatonin levels, suggestive advances in skeletal muscle circadian phase, and exploratory changes in circulating metabolites, lipids, and immune-cell gene expression linked to insulin sensitivity and lipid metabolism.

A key strength of the study is its randomized crossover design, which features tightly controlled light exposure, meals, and activity. However, the small sample size, short intervention duration, older 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 longer, larger, and more naturalistic real-world studies, particularly in working-age populations and real office environments.

Download your PDF copy now!