The participants stayed for 4.5 days in a room with a large window. From 8 a.m. to 5 p.m., they spent 9 hours daily exposed only to sunlight, without artificial lighting. They continued their usual diabetes medication and consumed three meals a day designed to maintain their weight. Sleep times and exercise were also kept consistent.

Under the same conditions, the participants spent another 4.5 days in a windowless room with only artificial lighting. The artificial light experiment was conducted a month after the sunlight exposure.

Analysis showed that blood sugar levels remained within the normal range for a longer duration (50% of the total time) when participants were exposed to natural sunlight. In contrast, under artificial lighting, their blood sugar stayed normal for only about 43% of the time.

◇Sunlight Enhances Fat Burning

Though the difference may seem small, Professor Hoeks emphasized, “Cumulative time spent outside the normal blood sugar range can increase the risk of diabetes complications like heart disease. This difference is meaningful.”

The team attributed the variation to light-sensitive cells in the eyes, which play a key role in regulating metabolic cycles. These cells are particularly responsive to short wavelengths (blue light spectrum) abundant in natural sunlight.

Exposure to sunlight improves circadian rhythms, enhancing fat burning and energy use. At night, increased melatonin production promotes better sleep. The study also noted that muscle cell clocks functioned more stably under these conditions.

Professor Glen Jeffrey from the Department of Visual Neuroscience at University College London, who reviewed the study, called it “a demonstration of how crucial daylight is to our bodies,” while noting, “Large-scale clinical studies are still needed.”