Long-term exposure to bright, artificial night light linked to increased risk of overweight and obesity in schoolchildren. Digest
Childhood obesity is a growing concern with multiple contributing factors, and perhaps, exposure to artificial light at night is among the most important. To investigate this, researchers in China followed more than 218,000 schoolchildren over a 15-year period.
Each year, the students underwent health examinations that included body mass index measurements. At the same time, satellite data provided yearly measurements of how bright the outdoor night sky was in the area around each school. Children were then grouped based on long-term trends over the study period: areas that stayed consistently bright, areas that brightened slowly, areas that brightened rapidly, and areas where brightness declined. Because the rapidly brightening pattern had the lowest baseline light, it served as the reference group for all comparisons.
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The study revealed distinct differences across the light exposure groups:
- Children attending schools in areas that stayed consistently bright had the highest risk. Boys had a 67% higher rate of becoming overweight or obese, and girls had a 56% higher rate.
- In areas that brightened slowly, the risk was elevated but not statistically significant.
- Areas where brightness declined showed no meaningful difference from the reference.
- Among children who began at a normal or low weight, the consistently bright group recorded 9.95 new cases of overweight or obesity per 1,000 person-years, compared with 6.10 in the rapidly brightening group and 8.04 in the slowly brightening group.
- On a population level, long-term exposure to consistently bright night-time environments accounted for about 40 percent of the risk in boys and 36 percent in girls.
Prolonged exposure to artificial light disrupts circadian rhythms, the internal clocks that regulate daily cycles of hormones such as melatonin and cortisol. When these rhythms are disturbed, appetite, metabolism, and fat storage can be affected in ways that promote weight gain. Another possible mechanism may involve brown fat, a tissue that normally burns energy to generate heat but may become less active under constant night-time light. The differences between boys and girls may reflect a mix of biological sensitivity and behavioral factors such as screen use. Finally, the stronger risks in the consistently bright and slowly brightening groups may point to the impact of longer cumulative exposure, whereas more recent increases in brightness, as in the rapidly brightening group, may not have had the same effect.
Conclusion:
Children who attended schools in brighter night-time environments were more likely to gain excess weight over the school years. The study was limited because light levels were estimated from school locations, not individual homes, and detailed information on diet, activity, or puberty was unavailable. Moreover, while the analysis adjusted for broad regional differences, it cannot be ruled out that environmental and socioeconomic changes occurring alongside rising night-time light influenced the results. Still, the findings highlight night-time light as a potential and under-recognized factor in childhood overweight and obesity. Learn more about how to improve sleep in episode #45 featuring Dr. Matthew Walker.