The Circadian system enables the body to anticipate environmental changes. In mammals, it is regulated by a central clock in the brain known as the suprachiasmatic nucleus. Light is the main signal that sets this clock. This central clock synchronizes the peripheral cellular clock, which affects up to 40% of gene expression and plays a key role in metabolism. Disruption of circadian rhythms is associated with type 2 diabetes (T2D) and insulin resistance. Modern indoor lifestyles reduce exposure to natural daytime light while increasing exposure to evening artificial light, potentially impairing metabolic health. A recent study published in Cell Metabolism aimed to examine whether indoor natural daylight compared with typical artificial office lighting improves 24-hour metabolism, biological clock function, and glucose control in people with T2D.
In this randomized crossover clinical trial, a total of 13 adults (mean age = 70±6 years, female = 8, male = 5, body mass index [BMI] = 30.1±2.3 kg/m2, HbA1c = 6.8±1.0%) with T2D were included. Each participant completed two 4.5-day intervention periods. During each period, participants were exposed to either natural daylight via large windows or constant artificial lighting during office hours from 08:00 to 17:00. The primary endpoint was overall glucose control, which is measured by continuous glucose monitoring across the intervention period. Secondary endpoints were plasma metabolites, clock gene expression in skeletal muscles, meal-related substrate handling, and 24-hour energy metabolism. Moreover, exploratory endpoints were body temperature, multi-omic metabolic markers, mood, melatonin levels, sleep, and cardiovascular measures.
Continuous glucose monitoring (CGM) data showed that there was no significant difference in mean glucose levels between natural and artificial lighting (7.4 ± 1.3 mmol/L vs 7.8 ± 1.3 mmol/L, p = 0.368). But overall time spent (4.5 days) in the normal range was higher with natural light compared to artificial light (50.9% ± 21.5% vs 43.3% ± 23.8%, p = 0.036). Natural light also reduced the 24-glucose rhythm amplitude (p = 0.010), which correlated with less time spent in the normal glucose range (p = 0.048).
Respiratory exchange ratio (RER) after dinner (natural: 0.852 ± 0.03 vs artificial: 0.849 ± 0.027, p = 0.717) and during sleep (0.829 ± 0.031 vs 0.826 ± 0.023, p = 0.763) were similar among participants on day 3. Whereas on day 4, RER was lower under natural light (p = 0.029), enhanced fat oxidation (p = 0.023), reduced carbohydrate oxidation (p = 0.034), especially at 13:00 hr (0.802 ± 0.028 vs 0.833 ± 0.028, p = 0.024).
During the mixed-meal tolerance test on day 5, energy expenditure peaked at 30-60 min with no condition differences (p < 0.001). RER remained lower under natural daylight compared to artificial light (0.834 ± 0.023 vs 0.845 ± 0.013) with trends towards lower carbohydrate oxidation (p = 0.059) and higher fat oxidation (p = 0.054). Plasma metabolites reported diurnal variation but no overall condition effects; only free fatty acids tended to be higher with natural light, with p = 0.061.
Furthermore, evening melatonin area under curve (AUC) was found to be higher under natural light compared to artificial light (755 ± 432 pg/mL × min vs 650 ± 374 pg/mL × min, p = 0.029, n = 13) whereas mean dim light melatonin onset (DLMO) timing was unchanged (20:35 ± 01:01 h vs 20:42 ± 01:18 h, p = 0.47, n = 9). Muscle biopsies showed higher biological clock gene expression of Per1, Per2, and Cry upon natural light compared to artificial light, with p = 0.01, p = 0.066, and p = 0.021, respectively. Multi-omic analyses identified light-related shifts in metabolites, lipids, and transcripts linked to improved glucose regulation.
Despite its robust crossover design, the study was limited by a small sample size (n = 13), short intervention duration, older participants, seasonal timings, limited generalizability, and modest predictive power of exploratory multi-omic analyses.
In conclusion, natural daylight exposure during office hours improved glucose control and increased fat oxidation in individuals with T2D, highlighting the importance of light-environment interactions in metabolic health.
Reference: Harmsen JF, Habets I, Biancolin AD, et al. Natural daylight during office hours improves glucose control and whole-body substrate metabolism. Cell Metab. 2025. doi:10.1016/j.cmet.2025.11.006
