Natural daylight has a beneficial impact on the metabolic circadian rhythm of patients with type 2 diabetes

A blog by Ivo Habets

“Natural daylight during office hours improves glucose control and whole-body substrate metabolism”

The circadian timing system regulates many processes in the human body. While it is best known for controlling our sleep–wake cycle, metabolism also follows a strong daily rhythm. This includes processes such as glucose regulation, insulin sensitivity, and substrate use, which vary across the day in a coordinated manner. The main entrainment signal for the circadian rhythm is natural light, rich in blue-white light, detected by retinal cells in our eyes.

Nowadays, however, much of our daytime is spent indoors under constant artificial lighting that differs substantially from natural daylight in both intensity and spectral composition. It is also known that people with metabolic diseases such as type 2 diabetes show disturbances in their circadian metabolic rhythms. Until now, it has been unknown how light exposure affects their main circadian rhythm and the regulation of metabolism as a consequence.

A recent study by Harmsen et al. investigated the effects of daytime light exposure on metabolic outcomes in people with type 2 diabetes. Participants were exposed to two different lighting conditions for five consecutive days. In one condition, they were exposed only to artificial indoor light. In the other condition, they were exposed to natural daylight behind a window between 8:00 and 17:00. Other factors, such as meals and daily routines, were controlled.

The results showed that glucose control was better during the natural daylight condition, as indicated by more time spent within normal glucose levels when participants were in the natural daylight condition. However, there was no difference in the mean glucose levels between the two conditions. Nevertheless, this improvement in glucose stability is clinically meaningful, as fluctuations in glucose are associated with metabolic complications.

In addition, substrate use was assessed at multiple time points at the end of the week. Participants relied more on fat metabolism in the natural daylight condition compared to the artificial light condition. This effect was most pronounced when the intensity of natural light was brightest. These findings suggest that daylight may strengthen the daily rhythm of macronutrient utilisation.

During the last evening of the week, melatonin levels were also measured. While the onset of melatonin secretion did not occur earlier, melatonin concentrations after onset were higher in the natural daylight condition.

This study demonstrates that metabolism is influenced by the circadian system and that natural light acts as a key regulator. Importantly, natural daylight exposure should not be viewed as a treatment or cure for metabolic disease; rather, it can support circadian alignment through daytime light exposure and therefore plays a valuable supportive role in improving disrupted metabolic rhythms.

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