title: Positive effect of daylight exposure on nocturnal urinary melatonin excretion in the elderly: a cross-sectional analysis of the HEIJO-KYO study.
abstract: BACKGROUND
  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.
  
  
  METHODS
  A cross-sectional study was conducted in 192 elderly individuals (mean age, 69.9 yr).
  
  
  METHODS
  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.
authors.affiliation: Department of Indoor Environmental Medicine, Nara Medical University School of Medicine, 840 Shijocho, Kashiharashi, Nara 634-8521, Japan. obayashi@naramed-u.ac.jp
authors.forename: Kenji
authors.initials: K
authors.lastname: Obayashi
authors.forename: Keigo
authors.initials: K
authors.lastname: Saeki
authors.forename: Junko
authors.initials: J
authors.lastname: Iwamoto
authors.forename: Nozomi
authors.initials: N
authors.lastname: Okamoto
authors.forename: Kimiko
authors.initials: K
authors.lastname: Tomioka
authors.forename: Satoko
authors.initials: S
authors.lastname: Nezu
authors.forename: Yoshito
authors.initials: Y
authors.lastname: Ikada
authors.forename: Norio
authors.initials: N
authors.lastname: Kurumatani
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D002940:Circadian Rhythm / Q000502:physiology*; D003430:Cross-Sectional Studies; D004777:Environment; D005260:Female; D006801:Humans; D008027:Light; D008297:Male; D008550:Melatonin / Q000652:urine*; D008875:Middle Aged; D012621:Seasons; D012890:Sleep / Q000502:physiology; D013472:Sunlight
chemical_list: D008550:Melatonin