Exposure to natural daylight, not artificial lighting, in an office setting improved glycemia and other metabolic parameters in people with type 2 diabetes (T2D), new research found. 

“Since it is cheap, easy, and risk-free — apart from direct sun exposure during the summer perhaps — I would recommend anyone to increase exposure to natural light during the day,” said lead author Joris Hoeks, PhD, associate professor in the Department of Nutrition and Movement Sciences at Maastricht University, Maastricht, Netherlands.

“Even better to go outside and combine it with physical activity, a proven recipe for improving metabolic health,” he told Medscape Medical News. 

Daylight and Metabolic Health

Disturbances in the circadian system or its alignment to the external environment may contribute to metabolic disease, with the day-night cycle being one of the main circadian alignment factors. Previous studies have found that night owls may be at increased risk for T2D, as might those with high light exposure during the night hours.

This randomized crossover study, published January 6, in Cell Metabolism, is the first to compare the effect of indoor natural daylight exposure with a typical artificial light environment under controlled conditions on glycemia and 24-hour substrate metabolism in people with T2D. 

A total of 13 individuals with T2D each spent 4.5 business days, from 8:00 AM to 5:00 PM, in an office either facing a large window or in an artificially lit windowless room. After a 4-week washout period, they were switched to the other lighting condition. There were eight women and five men in the study with a mean age of 70 years, mean fasting plasma glucose levels of 8.1 mmol/L, and an average A1c of 6.8%.

The primary outcome, average glucose control as assessed using continuous glucose monitoring, available for 10 participants, did not differ significantly between natural and artificial light conditions (7.4 mmol/L and 7.8 mmol/L, respectively; P = .368). However, the overall time spent in normal range (4.4-7.2 mmol/L) was significantly higher with natural vs artificial light (50.9% vs 43.3%, P = .036). The trend was similar using the American Diabetes Association’s definition of time in range (3.9-10.0 mmol/L), with 83% for natural light vs 78.6% for artificial light (P = .082).

In addition, lower whole-body carbohydrate oxidation (assessed by indirect calorimetry) and fat oxidation rates were higher during waking hours with natural light. Consistent with higher fat oxidation, plasma free fatty acid levels were also higher following a liquid mixed meal challenge in natural light than in artificial office lighting.

“Although the effects we observed in our small scale, proof-of-principle study were not large, they clearly show that daytime exposure to natural light does impact metabolism and glucose control,” Hoeks said.

Salivary samples also showed higher late-evening levels of melatonin with daytime exposure to natural light. “In our view, this is an indication that the central circadian system was influenced by the natural light regime, and may hence be involved in mediating the observed metabolic effects,” he noted.

“I have a great interest in the 24-hour regulation of human metabolism in relation to insulin resistance and type 2 diabetes and believe that lifestyle interventions aimed at enhancing the daily rhythmicity of the circadian timing system hold great promise to improve metabolic health and glucose homeostasis,” Hoeks told Medscape Medical News.

“In this context, we are currently also studying the effects of timed lifestyle interventions such as morning vs afternoon exercise training and timed nutritional interventions,” he added.

Hoeks and co-authors declared having no disclosures. 

Miriam E. Tucker is a freelance journalist based in the Washington DC area. She is a regular contributor to Medscape Medical News, with other work appearing in the Washington Post, NPR’s Shots blog, and Diatribe. She is on X @MiriamETucker and BlueSky @miriametucker.bsky.social.