Want Better Blood Sugar? Try Sitting Near a Window
Key findings: 60-second read
- Natural daylight improves blood sugar control - people with type 2 diabetes spent significantly more time with normal blood sugar levels when exposed to natural daylight during office hours (4.5 days).
- The body burned more fat during the day - natural light shifted metabolism toward higher fat oxidation, meaning the body used more fat for energy rather than carbohydrates.
- Evening melatonin levels were higher - natural daylight during the day led to higher melatonin (the sleep hormone) in the evening, which may help with better sleep and circadian alignment.
- Natural light affected the body's internal clocks - muscle cells showed changes in clock gene expression, and the timing of the body's 24-hour rhythms shifted slightly earlier.
- The built environment matters for metabolic health - with 80-90% of time spent indoors, this study raises important questions about office lighting design and metabolic disease risk.
Most of us spend the majority of our time indoors. In fact, research suggests we spend 80% to 90% of our time inside buildings - at home, in offices, in cars. And during those hours, we are almost always under artificial lighting.
But what if the type of light we are exposed to during the day actually affects our health? What if something as simple as sitting near a window could help control blood sugar?
A new study published in Cell Metabolism (Harmsen et al., 2026) set out to answer exactly this question. The researchers took 13 older adults with type 2 diabetes and exposed them to either natural daylight through large windows or standard artificial office lighting for 4.5 days. They measured everything: blood sugar, fat burning, hormone levels, muscle clocks, and even gene expression.
The findings are interesting and may have important implications for anyone with type 2 diabetes, prediabetes, or even those simply wanting to optimise their metabolic health.
"Because 80%–90% of our time is spent indoors and daylight is the main synchronizer of the central biological clock, the chronic lack of daylight is increasingly considered as a risk factor for metabolic diseases, such as type 2 diabetes."
On this page
Why daylight matters for your body clock
Every cell in your body has an internal clock. These clocks cycle through a 24-hour rhythm, telling your body when to be awake, when to sleep, when to digest food, and when to burn fat.
When light enters your eyes, it signals to your brain that it is daytime. It then synchronises all the other "clocks" in your body.
Here is the problem: we spend almost all our time indoors under artificial light. Artificial light is dimmer than daylight, has a different colour spectrum, and does not change naturally throughout the day. Over time, this may essentially confuse our brain as to what time of day it is, leading to poorer sleep, a more disrupted metabolism, and the potential for increased risk of diseases like type 2 diabetes.
This study asked a simple question: if we bring natural daylight back into people's lives during office hours, does it improve their metabolic health?
The circadian connection
"The main circadian alignment factor is the day-night cycle, i.e., natural daylight exposure during the day and darkness during the night. Yet the current indoor lifestyle is characterised by spending 80%–90% of time indoors under predominantly constant artificial lighting of low illumination levels."
Study design: 13 people, 4.5 days, two lighting conditions
Thirteen older adults with type 2 diabetes (average age 70 years) participated in this randomised crossover trial. Each person went through two separate 4.5-day periods:
- Natural light condition: During office hours (8am-5pm), participants sat in a room with large windows facing outside, receiving natural daylight.
- Artificial light condition: In the same room but separated by a lightproof divider, participants sat under standard artificial office lighting (about 300 lux - typical for an office).
The order was randomised, and there was a 4-week break between conditions. Participants wore special orange-tinted glasses whenever they left the room to block any natural light from affecting the artificial light condition.
Throughout the study, researchers measured blood sugar continuously (using a glucose monitor), fat and carbohydrate burning (using indirect calorimetry), melatonin levels, muscle biopsies, and blood samples for detailed molecular analysis.
"Participants were fed the same meals at the same times, had the same sleep schedule, and the same physical activity throughout both conditions. The only difference was the type of light they were exposed to during office hours."
Blood sugar control: more time in normal range
The primary finding of this study is encouraging. People exposed to natural daylight spent significantly more time with normal blood sugar levels (4.4-7.2 mmol/L) compared to when they were under artificial light.
Specifically, during natural light, participants spent 50.9% of their time in the normal glucose range, compared to only 43.3% under artificial light.
Why does this matter? Time spent in the normal glucose range is a clinically important outcome. Studies have shown that less time in the normal range is associated with higher risk of death in people with type 2 diabetes.
The researchers also modelled the data and found that natural light led to lower 24-hour fluctuations in blood sugar. In other words, blood sugar levels were more stable throughout the day, with smaller spikes and dips.
What this means
"The time spent in the normal glucose range over the entire 4.5 days was significantly higher upon natural light (50.9% vs. 43.3%, p = 0.036). This improvement in glucose control is similar to what is seen with time-restricted eating or exercise programs in people with type 2 diabetes."
Fat burning: shifting metabolism toward fat
During the day, the researchers measured how much fat and carbohydrate people were burning. They found a clear shift under natural light: the body relied more on fat for energy and less on carbohydrates.
This is known as increased fat oxidation. People under natural light had a lower respiratory exchange ratio (RER), which means they were burning a higher proportion of fat relative to carbohydrates.
This effect was observed throughout the day and also during a mixed meal tolerance test (a standardised test where participants drink a liquid meal and researchers measure how their body handles it).
Importantly, total energy expenditure (how many calories were burned) did not change between conditions. So the shift was not about burning more calories overall, but about what fuel the body was using - more fat, less sugar.
For people with type 2 diabetes, this is a favourable shift. It suggests improved metabolic flexibility - the ability to switch between burning carbohydrates and fats depending on what is available.
Melatonin: higher evening levels after daylight
Melatonin is the hormone that helps you fall asleep. It rises in the evening as it gets dark, signalling to your body that it is time to wind down.
In this study, people who were exposed to natural daylight during the day had higher melatonin levels in the evening (between 9pm and 11pm) compared to those under artificial light.
This is important because lower evening melatonin has been linked to poorer sleep quality and disrupted circadian rhythms. The fact that natural daylight increased evening melatonin suggests it may help set the body's internal clock more effectively.
The timing of melatonin onset (the time when melatonin starts to rise) did not change between conditions. But the amount of melatonin released after that point was higher with natural light.
"Melatonin levels from 21:00-23:00 h were higher in natural compared with artificial office light (p = 0.029), indicating higher melatonin release after DLMO upon natural light."
Blood markers: what changed at the molecular level
The researchers also analysed blood samples for thousands of different molecules - metabolites (small molecules involved in metabolism), lipids (fats), and gene expression in immune cells.
Several interesting patterns emerged. Under natural light, levels of cholic acid (a bile acid) were higher, while ceramides (a type of fat linked to insulin resistance) tended to be lower. Ceramides are known to be elevated in people with type 2 diabetes, so lower levels are a good sign.
Certain types of lipids called LPEs (lysophosphatidylethanolamines) were also higher under natural light. These have been linked to higher insulin sensitivity.
Machine learning analysis - a type of artificial intelligence - identified a combination of metabolites, lipids, and gene transcripts that could distinguish between natural and artificial light conditions with 56% accuracy. While not perfect, this suggests there is a real biological signature associated with natural light exposure.
A clinical perspective
"These findings reveal a distinct multi-omic signature linked to natural light exposure, characterised by alterations in metabolites, lipids, and transcripts possibly underlying the improved glucose control."
From my clinical experience: what this means for you
As a physiotherapist, I do not treat diabetes directly. But I work with many people who have type 2 diabetes, prediabetes, or metabolic syndrome. And I am always looking for simple, non-pharmacological interventions that can help my patients - especially ones that cost nothing and avoid side effects.
Getting more natural daylight fits that description perfectly.
This study suggests that something as simple as sitting near a window during the day can improve blood sugar control, shift metabolism toward fat burning, and positively affect your body's internal clocks. And it only took 4.5 days to see measurable improvements.
Here is what I would recommend based on this research:
1. Get morning light. The earlier in the day you get natural light, the stronger the signal to your body clock. Aim for at least 30 minutes of daylight exposure within the first hour of waking.
2. Work near a window if possible. If you have a desk job, position your desk near a window. If that is not possible, take your breaks outside. Even 10-15 minutes of daylight exposure during the day may help.
3. Reduce artificial light at night. The study also highlights the importance of darkness in the evening. Bright artificial light at night can confuse your body clock and reduce melatonin production. Dim your lights in the evening, use blue-light-blocking glasses, and avoid screens before bed.
4. Consider your built environment. If your workplace has poor natural light, consider talking to your employer about lighting design. This study suggests that office lighting is not just about comfort - it may actually affect metabolic health.
5. Combine daylight with other healthy habits. The participants in this study had standardised meals, sleep, and physical activity. Daylight is not a magic bullet, but it appears to be a meaningful contributor to metabolic health. Combine it with good nutrition, regular exercise, and quality sleep for the best results.
Of course, this study was small (13 participants) and lasted only 4.5 days. A lot more research is needed. But the findings are compelling, and the intervention is free and low-risk. If you have type 2 diabetes or are at risk, getting more natural daylight is a simple step worth trying.
A clinical perspective
"Our findings provide a strong rationale for future research to focus more on the interaction of (natural) light exposure and metabolic health. This study also highlights the often unnoticed impact of the built environment on our health and raises further concerns about the prevalence of office environments with poor (natural) daylight access."
Study strengths and limitations
Strengths:
- Randomised crossover design (each person served as their own control)
- Standardised meals, sleep, and physical activity (eliminating confounding factors)
- Comprehensive measurements: continuous glucose monitoring, indirect calorimetry, muscle biopsies, blood multi-omics
- Real-world comparison (natural daylight vs. realistic artificial office lighting)
Limitations:
- Small sample size (n=13) - limits generalisability and statistical power
- Short duration (4.5 days) - longer-term effects unknown
- Older participants (average age 70) - may not generalise to younger adults
- All participants had type 2 diabetes - effects on healthy individuals unknown
- Conducted in summer (April-October) - seasonal effects not examined
- No objective sleep measurement - only subjective questionnaires
Conclusions: a simple intervention with meaningful metabolic benefits
This study provides evidence that exposure to natural daylight during office hours improves blood sugar control and shifts whole-body metabolism toward higher fat oxidation in older adults with type 2 diabetes. These benefits occurred after just 4.5 days, with no other changes to diet, exercise, or sleep.
The improvements were accompanied by higher evening melatonin levels, changes in muscle clock gene expression, and distinct alterations in blood metabolites and lipids.
The authors conclude: "Natural daylight exposure has a positive metabolic impact on individuals with type 2 diabetes and could support the treatment of metabolic diseases."
For the millions of people who spend their days in artificially lit offices, this research suggests that something as simple as sitting near a window could have meaningful health benefits. It also raises important questions about how we design our built environment and whether current office lighting standards are adequate for metabolic health.
One key insight from this research
"People with type 2 diabetes who were exposed to natural daylight during office hours spent significantly more time with normal blood sugar levels (50.9% vs 43.3%) and burned more fat for energy compared to when they were under artificial lighting. Evening melatonin levels were also higher, and muscle cell clocks shifted earlier by about 45 minutes. All this after just 4.5 days."
Frequently asked questions
Does sunlight through a window count, or do I need to be outside?
In this study, natural daylight was provided through large windows, and participants were sitting indoors. So yes, light through a window does count. However, windows block some of the UV spectrum, and the light intensity is lower than being directly outside. If you can get outside, even for a few minutes, that is even better. But sitting near a window is a good start.
How much daylight do I need?
In this study, participants were exposed to natural daylight from 8am to 5pm (9 hours). However, the beneficial effects likely occur with less exposure. Aim for at least 30-60 minutes of natural light exposure during the morning and early afternoon. Even small amounts are better than none.
Does this apply to people without diabetes?
This study only included people with type 2 diabetes, so we cannot say for certain that the same effects occur in healthy individuals. However, the mechanisms (circadian alignment, melatonin production, fat oxidation) are relevant to everyone. Other studies have shown that light exposure affects metabolism in healthy people as well.
Can I use bright artificial light instead of natural daylight?
The study compared natural daylight to standard artificial office lighting (300 lux). Other studies have used bright artificial light (1,250-4,000 lux) and found effects on glucose metabolism, but sometimes in the opposite direction (increasing postprandial glucose). Natural daylight has a different wavelength spectrum that changes dynamically throughout the day. It is not clear that bright artificial light would produce the same benefits.
What about people who work night shifts?
Night shift workers are a special case. This study did not include shift workers. People who work at night and sleep during the day have chronically misaligned circadian rhythms, which is associated with higher risk of metabolic disease. The principles may still apply (getting bright light during your "day" and darkness during your "night"), but the timing would be shifted.
This study is a powerful reminder that our health is shaped by more than just diet and exercise. The environment we live in - including the type of light we are exposed to - matters.
If you spend your days in an artificially lit office, consider this: moving your desk closer to a window, taking your breaks outside, or even just stepping out for 10 minutes of morning sunlight could be a simple, free, and effective way to support your metabolic health.
As a physiotherapist, I help people move better, feel better, and manage chronic conditions. While I do not treat diabetes directly, I know that blood sugar control, fat metabolism, and circadian rhythms are deeply connected to musculoskeletal health and overall wellbeing.
If you are living with type 2 diabetes, prediabetes, or metabolic syndrome, I would encourage you to speak to your doctor or trusted health professional about the results of this study. It is not a replacement for medical treatment, but may be a simple, low-risk addition.
I see patients in Port Macquarie and via telehealth for comprehensive movement and lifestyle advice. If you would like to discuss how to integrate simple changes like light exposure into your health plan, I am here to help.
- Grant
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Disclaimer: This information is for educational purposes and does not replace individualised medical advice. Always consult a qualified healthcare professional for your specific situation. This blog post summarises a published research study (Harmsen JF, Habets I, Biancolin AD, et al. Natural daylight during office hours improves glucose control and whole-body substrate metabolism. Cell Metab. 2026;38(1); the original source should be consulted for full methodological details.
