The commentary with wide perspectives by Hasler (1) provides a precise assessment including both the strengths and limitations of our prospective cohort study (2). We agree with his conclusions that our data raise a number of questions about the link between exposure to light at night (LAN) and the risk of depression. Although previous animal studies have reported that 5-lux LAN increases depressive behavior (3, 4), the effects of such low-level LAN on human brain function, including mood and sleep, have not been well established. As described in our discussion, LAN levels observed in the present study might be higher than those used in animal studies (3, 4); in our study, LAN intensities in groups with an average exposure of ≥5 lux and ≥10 lux were 12.4 and 19.4 lux, respectively (2). Further interventional studies testing the effects of reducing LAN exposure on mood, as well as experimental studies exploring the biological mechanisms underlying the association between low-level LAN and depressive symptoms, are needed. The most important strength of our current study was objective measurement of LAN intensity using a bedside light meter. Most previous studies have assessed indoor LAN levels using a self-reported questionnaire (5) or outdoor LAN levels using satellite data (6). However, self-reported indoor LAN levels had not yet been validated with objective measurement, and outdoor LAN levels are surrogates for an individual LAN exposure. For larger-scale epidemiologic studies, indoor self-reported LAN levels should be validated with objectively measured data. The second strength of our study was its longitudinal design using multivariable methods to adjust for confounders, which could reduce the possibility that the LAN exposure might be a reflection of depression-associated behaviors and indicated that LAN exposure might be a cause of the incidence of depressive symptoms. Indeed, the depressive score evaluated by questionnaire might be above or below the cutoff value over the short term; therefore, a long-term study considering such unstable outcomes should be conducted. We previously reported the moderate day-to-day reproducibility of average LAN exposure, with correlation coefficients between 0.66 and 0.70 and a fair seasonal reproducibility of 0.45 (7, 8). In the current study, LAN exposure was measured for only 2 nights; thus, an amplitude of LAN intensity has been focused. However, in a future study, multiple measurements of LAN exposure over a longer period, such as 1 week, would enable the evaluation of a more accurate individual LAN exposure. In addition, multiple measurements over time would allow an analysis of fluctuations in LAN exposure, which might be important for circadian physiology, as shown in studies on social jet lag and circadian activity rhythms (9, 10). ACKNOWLEDGMENTS Author affiliations: Department of Community Health and Epidemiology, Nara Medical University School of Medicine, Nara, Japan (Kenji Obayashi, Keigo Saeki, Norio Kurumatani). This work was supported by research funding from the Department of Indoor Environmental Medicine, Nara Medical University; JSPS KAKENHI (grants 24790774, 22790567, 25860447, 25461393, 15H04776, and 15H04777); Mitsui Sumitomo Insurance Welfare Foundation; Meiji Yasuda Life Foundation of Health and Welfare; Osaka Gas Group Welfare Foundation; Japan Diabetes Foundation; Daiwa Securities Health Foundation; Japan Science and Technology Agency; YKK AP Inc.; Ushio Inc.; Nara Prefecture Health Promotion Foundation; Nara Medical University Grant-in-Aid for Collaborative Research Projects; Tokyo Electric Power Company; EnviroLife Research Institute Co., Ltd.; and Sekisui Chemical Co., Ltd. Conflict of interest: K.O. and K.S. received research grant support from YKK AP Inc., Tokyo Electric Power Company, EnviroLife Research Institute Co. Ltd., and Sekisui Chemical Co. Ltd. N.K. reported no conflicts. REFERENCES 1 Hasler BP. Invited commentary: “Bedroom light exposure at night and the incidence of depressive symptoms: a longitudinal study of the HEIJO-KYO cohort”. Am J Epidemiol . 2018; 187( 3): 435– 438. 2 Obayashi K, Saeki K, Kurumatani N. Bedroom light exposure at night and the incidence of depressive symptoms: a longitudinal study of the HEIJO-KYO cohort. Am J Epidemiol . 2018; 187( 3): 427– 434. 3 Bedrosian TA, Weil ZM, Nelson RJ. Chronic dim light at night provokes reversible depression-like phenotype: possible role for TNF. Mol Psychiatry . 2013; 18( 8): 930– 936. Google Scholar CrossRef Search ADS PubMed 4 Fonken LK, Kitsmiller E, Smale L, et al. . Dim nighttime light impairs cognition and provokes depressive-like responses in a diurnal rodent. J Biol Rhythms . 2012; 27( 4): 319– 327. Google Scholar CrossRef Search ADS PubMed 5 McFadden E, Jones ME, Schoemaker MJ, et al. . The relationship between obesity and exposure to light at night: cross-sectional analyses of over 100,000 women in the Breakthrough Generations Study. Am J Epidemiol . 2014; 180( 3): 245– 250. Google Scholar CrossRef Search ADS PubMed 6 Koo YS, Song JY, Joo EY, et al. . Outdoor artificial light at night, obesity, and sleep health: cross-sectional analysis in the KoGES study. Chronobiol Int . 2016; 33( 3): 301– 314. Google Scholar CrossRef Search ADS PubMed 7 Obayashi K, Saeki K, Iwamoto J, et al. . Positive effect of daylight exposure on nocturnal urinary melatonin excretion in the elderly: a cross-sectional analysis of the HEIJO-KYO study. J Clin Endocrinol Metab . 2012; 97( 11): 4166– 4173. Google Scholar CrossRef Search ADS PubMed 8 Obayashi K, Saeki K, Iwamoto J, et al. . Effect of exposure to evening light on sleep initiation in the elderly: a longitudinal analysis for repeated measurements in home settings. Chronobiol Int . 2014; 31( 4): 461– 467. Google Scholar CrossRef Search ADS PubMed 9 Roenneberg T, Allebrandt KV, Merrow M, et al. . Social jetlag and obesity. Curr Biol . 2012; 22( 10): 939– 943. Google Scholar CrossRef Search ADS PubMed 10 Tranah GJ, Blackwell T, Stone KL, et al. . Circadian activity rhythms and risk of incident dementia and mild cognitive impairment in older women. Ann Neurol . 2011; 70( 5): 722– 732. Google Scholar CrossRef Search ADS PubMed © The Author(s) 2018. Published by Oxford University Press on behalf of the Johns Hopkins Bloomberg School of Public Health. All rights reserved. For permissions, please e-mail: firstname.lastname@example.org.
American Journal of Epidemiology – Oxford University Press
Published: Mar 1, 2018
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