Positive Effect of Daylight Exposure on Nocturnal Urinary Melatonin Excretion in the Elderly: A Cross-Sectional Analysis of the HEIJO-KYO Study

Context: Melatonin is involved in a variety of diseases, including cancer, insomnia, depression, dementia, hypertension, and diabetes; its secretion is influenced by environmental light. Although daylight exposure increases nocturnal melatonin secretion in a controlled laboratory setting, whether it increases nocturnal melatonin secretion in an uncontrolled daily life setting remains unclear. Objective: We aimed to determine the association between daylight exposure in an uncontrolled daily life setting and urinary 6-sulfatoxymelatonin excretion. Design and Participants: A cross-sectional study was conducted in 192 elderly individuals (mean age, 69.9 yr). Measures: We measured ambulatory daylight exposure using a wrist light meter in two 48-h sessions; furthermore, we measured overnight urinary 6-sulfatoxymelatonin excretion, an index of melatonin secretion, on the second night of each session. Results: The median duration of daylight exposure of at least 1000 lux was 72 min (interquartile range, 37–124). Univariate linear regression analysis showed marginal to significant associations between log-transformed urinary 6-sulfatoxymelatonin excretion and age, current smoking status, benzodiazepine use, day length, log-transformed duration of daylight exposure of at least 1000 lux, and daytime physical activity. In a multivariate model, log-transformed duration of daylight exposure of at least 1000 lux was significantly associated with log-transformed urinary 6-sulfatoxymelatonin excretion (regression coefficient, 0.101; 95% confidence interval, 0.003–0.199; P = 0.043). Furthermore, an increase in the duration of daylight exposure of at least 1000 lux from 37 to 124 min (25th to 75th percentiles) was associated with a 13.0% increase in urinary 6-sulfatoxymelatonin excretion (6.8 to 7.7 μg). Conclusions: Daylight exposure in an uncontrolled daily life setting is positively associated with urinary 6-sulfatoxymelatonin excretion in the elderly.