Natural daylight during office hours improved blood sugar stability and shifted fuel use toward fat in adults with type 2 diabetes. Digest
Modern life keeps most people indoors, often under artificial lighting that differs greatly from natural daylight. Because light is the main signal that sets the body's internal clock, limited daylight exposure could affect metabolism. A new study tested whether working by a window, rather than under typical office lighting, changes blood sugar control and how the body uses energy in people with type 2 diabetes.
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The study involved 13 older adults with type 2 diabetes who completed two tightly controlled, 4.5-day stays in a research facility. During office hours (8:00 a.m. to 5:00 p.m.), participants worked either next to large windows receiving natural daylight or under constant artificial office lighting set to about 300 lux, a common indoor brightness. Each person experienced both conditions, separated by at least four weeks. Meals, sleep times, and physical activity were kept the same.
- Average glucose levels were similar under both lighting conditions when measured continuously over the full study period, and insulin responses to a test meal also showed no differences.
- Daylight was associated with more stable blood sugar levels across the day. Participants spent more time (50.9% vs. 43.3%, daylight vs. artificial lighting) between 4.4 and 7.2 mmol/L, a range linked to better long-term health.
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Daylight shifted the body toward burning more fat. Breath-based measurements showed a lower respiratory exchange ratio (a measure that reflects fat versus carbohydrate use) without changes in total calories burned. This pattern was clear during the day and more modest after the standardized meal.
- Evening salivary melatonin levels were higher after daylight exposure, and muscle samples showed changes in genes that help run local biological clocks.
Natural daylight differs from typical office lighting in ways that matter for the body's internal clock, including higher intensity and gradual changes in brightness and color across the day. These features may provide clearer timing signals, which help coordinate daily patterns in hormone release, muscle metabolism, and energy use.
The results are promising but limited by the small sample size and the short intervention duration. Still, the findings suggest that something as simple as access to daylight at work could support blood sugar stability in people with type 2 diabetes. In this clip, I explain how HIIT, circadian-timed eating, and quality sleep improve metabolic health and reduce chronic disease risk.