Obesity and overweight are highly prevalent in individuals with bipolar disorder and are associated with a risk of developing not only physical but also mental problems. The current study aimed to determine the association between bedroom light exposure at night and obesity in individuals with bipolar disorder. This cross-sectional study enrolled 200 outpatients with bipolar disorder. The light intensity in the bedroom between bedtime and rising time was measured for seven consecutive nights using a portable photometer. Body mass index (BMI) was determined using self-reported height and weight, and obesity was defined as a BMI ≥ 25 kg/m2. The overall prevalence of obesity was 44%. In the multivariable logistic regression analysis adjusted for age, gender, use of psychiatric medications, sleep parameters, and physical activity, the odds ratio (OR) for obesity was significantly higher in the group exposed to an average light intensity ≥ 3 lux (n = 112) than in the group exposed to an average light intensity < 3 lux (n = 88) (OR, 2.13; 95% confidence interval, 1.19–4.21; P = 0.01). Furthermore, individuals exposed to an average light intensity ≥ 3 lux were significantly higher body weight (adjusted mean, 68.7 vs. 64.4 kg; P = 0.03) and BMI (adjusted mean, 25.6 vs. 24.2 kg/m2; P = 0.04) than those exposed to an average light intensity < 3 lux. A significant association was observed between bedroom light exposure at night and obesity in patients with bipolar disorder. Further longitudinal investigations are necessary to clarify this association.
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