In a randomized crossover clinical trial (NCT05263232), Harmsen et al. investigated the metabolic effects of natural daylight exposure during office hours in individuals with type 2 diabetes (T2D). Thirteen participants with T2D, virologically suppressed on treatment, were exposed to either natural daylight (facilitated through windows) or constant artificial office lighting for 4.5 consecutive days in a controlled setting, with interventions separated by a washout period.
Given that modern humans spend 80%–90% of their time indoors under artificial light, and daylight serves as the primary synchronizer of the central circadian clock, insufficient daylight has emerged as a potential risk factor for metabolic disorders like T2D. Continuous glucose monitoring demonstrated that natural daylight exposure led to significantly more time spent in the normal glucose range compared to artificial lighting, indicating improved glucose homeostasis. Indirect calorimetry revealed a marked shift in whole-body substrate metabolism, with reduced respiratory exchange ratio (RER), decreased carbohydrate oxidation, and increased fat oxidation during the awake period under daylight conditions.
Further analyses showed elevated evening melatonin levels, favorable alterations in 24-hour plasma multi-omic signatures (including glucose, free fatty acids, and triglycerides), and changes in circadian rhythms of skeletal muscle gene expression associated with natural daylight. The safety profile was comparable between conditions, with no serious adverse events reported. These results suggest that even short-term exposure to natural daylight during typical office hours can yield clinically meaningful metabolic benefits in T2D, potentially by better aligning peripheral clocks and enhancing circadian entrainment.
The study underscores the therapeutic potential of simple environmental interventions, such as maximizing workplace daylight access, to mitigate metabolic dysfunction linked to chronic artificial lighting. Larger, longer-term studies are warranted to confirm these findings and explore mechanisms, including impacts on insulin sensitivity and long-term glycemic control. Overall, this work positions inadequate daylight as a modifiable lifestyle factor in metabolic disease prevention and management.
Link: https://www.cell.com/cell-metabolism/fulltext/S1550-4131(25)00490-5
