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

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. In a randomized crossover design (NCT05263232), 13 individuals with type 2 diabetes were exposed to natural daylight facilitated through windows vs. constant artificial lighting during office hours for 4.5 consecutive days. Continuous glucose monitoring revealed that participants spent more time in the normal glucose range, and whole-body substrate metabolism shifted toward a greater reliance on fat oxidation during daylight. Primary myotubes cultured from skeletal muscle biopsies displayed a phase advance after daylight exposure. Multi-omic analyses revealed daylight-induced differences in serum metabolites, lipids, and monocyte transcripts. Our findings suggest that natural daylight exposure has a positive metabolic impact on individuals with type 2 diabetes and could support the treatment of metabolic diseases.