Abstract and Introduction
Abstract
There has been a worldwide epidemic of obesity in recent decades. In animal studies, there is convincing evidence that light exposure causes weight gain, even when calorie intake and physical activity are held constant. Disruption of sleep and circadian rhythms by exposure to light at night (LAN) might be one mechanism contributing to the rise in obesity, but it has not been well-investigated in humans. Using multinomial logistic regression, we examined the association between exposure to LAN and obesity in questionnaire data from over 100,000 women in the Breakthrough Generations Study, a cohort study of women aged 16 years or older who were living in the United Kingdom and recruited during 2003–2012. The odds of obesity, measured using body mass index, waist:hip ratio, waist:height ratio, and waist circumference, increased with increasing levels of LAN exposure (P < 0.001), even after adjustment for potential confounders such as sleep duration, alcohol intake, physical activity, and current smoking. We found a significant association between LAN exposure and obesity which was not explained by potential confounders we could measure. While the possibility of residual confounding cannot be excluded, the pattern is intriguing, accords with the results of animal experiments, and warrants further investigation.
Introduction
The prevalence of obesity in developed societies has been increasing for decades, and the obesity epidemic has become one of the most important global public health issues. Much emphasis has been placed upon changing well-documented factors that are known to contribute to weight gain, such as dietary choices and physical activity levels. However, the number of overweight and obese adults and children has continued to increase, and the potential effects of other factors are now also being considered.
Energy homeostasis is controlled through endogenous circadian rhythms that are regulated by light information, entraining an individual's behavior and physiology to the external night-day cycle. It is believed that changes in the light-dark exposure pattern or "inappropriate" light exposure can affect the circadian rhythm in such a way that the internal rhythms are desynchronized from both the external environment and internally with each other, which may impair sleeping behaviors and compromise metabolic processes. There is growing evidence from animal studies that disruption of circadian rhythms may lead to metabolic alterations and aspects of the metabolic syndrome, including obesity, and molecular studies have shown that metabolism is directly regulated by circadian genes. In humans, night-shift workers, whose usual patterns of sleeping and eating and light-dark phases are disturbed, are more likely to be obese than day-shift workers; epidemiologic studies have shown a negative association between sleep duration and both body mass index (BMI; weight (kg)/height (m)) and diabetes; and small experimental studies have shown that circadian misalignment through disruptions of sleeping and feeding patterns has adverse metabolic and cardiovascular consequences.
An increase in exposure to artificial light at night (LAN) has coincided with the increase in obesity and metabolic diseases over the last century, but whether this is coincidence or there is a causal connection has not been well investigated in humans. A recent cross-sectional study of 500 people in Japan found that elderly people sleeping in lighter rooms had higher body weight, waist circumference, and BMI; in that study, light exposure and obesity outcome variables were all objectively measured, although the BMI of participants was generally low (an average of 22.8). Thus, disruption of sleep and circadian rhythms might be one mechanism contributing to the worldwide rise in obesity and metabolic syndrome. We therefore examined the association between LAN exposure and obesity in data from over 100,000 women in a large United Kingdom cohort study.