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Increased nighttime, daytime light exposure may affect obesity parameters
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Adults exposed to higher light intensity at night are more likely to experience gains in BMI and waist-to-height ratio vs. those exposed to longer periods of darkness during the night, according to findings from a longitudinal study.
“In our study, nighttime light intensity and daytime light exposure had mutually independent effects on the obesity parameters,” Kenji Obayashi, MD, PhD, of the department of community health and epidemiology at Nara Medical University School of Medicine in Nara, Japan, and colleagues wrote. “Additionally, increased nighttime light and decreased daytime light are common exposures in modern society, suggesting high population attributable risks. Thus, changing daily light exposure patterns may be clinically significant for reducing the obesity risk.”
In a prospective, population-based study, Obayashi and colleagues analyzed data from Japanese adults aged at least 60 years participating in the HEIJO-KYO study between September 2010 and April 2014 (1,110 at baseline, 766 at follow-up; mean age, 72 years; median follow-up time, 21 months). Researchers visited homes and measured waist circumferences, body weight and height, and conduced face-to-face interviews. Participants wore a wrist light meter (Actiwatch 2, Respironics Inc.) to measure light exposure in the morning, evening and daytime for 2 consecutive days at 1-minute intervals; light exposures during the nighttime and during the first hour after bedtime and first hour before rising time were measured using a portable light meter (LX-28SD, Sato Shouji Inc.). Participants also kept sleep diaries and completed food frequency questionnaires. Researchers used the Pittsburgh Sleep Quality Index questionnaire to assess subjective sleep quality.
Mean waist-to-height ratio for the cohort was 0.536; median of mean light intensities in the morning, evening, daytime and nighttime were 345.9 lux, 24 lux, 337.2 lux and 0.7 lux, respectively.
After adjusting for confounders (caloric intake, physical activity and sleep/wake parameters), researchers found that exposure to evening light at a higher intensity (at least 100 lux) and for a longer period of time were associated with subsequent gains in waist-to-height ratio (P = .045 and .032, respectively). Daytime exposure to at least 500 lux for a longer period of time was associated with loss in waist-to-height ratio that persisted after adjustment (P = .021).
Nighttime exposure to higher light intensity (mean of at least 3 lux) was associated with gains in waist-to-height ratio in each statistical model, according to researchers (P = .01).
Evening or nighttime exposure to higher light intensity and evening exposure to a longer period of time was associated with gains in BMI, whereas BMI loss was associated with daytime exposure to at least 500 lux for a longer period of time and nighttime exposure to a longer time below the thresholds.
“Cumulative effects of light exposure on the obesity risk can be estimated from our results,” the researchers wrote. “Higher nighttime light intensity ( 3 vs. < 3 lux) and increased evening light intensity from the 25th to 75th percentiles (15.9–37 lux) corresponded to 10.2% and 4.5% for [waist-to-height ratio] gain, and 10% and 9.1% for BMI gain, respectively, over 10 years.
Conversely, increased daytime exposure to 500 lux and increased nighttime exposure to time < 3 lux from the 25th to 75th percentiles (40-129 minutes and 420-516 minutes, respectively) corresponded to 5.3% and 6.1% for [waist-to-height ratio] loss, and 6.7% and 7.1% for BMI loss, respectively, over 10 years.” – by Regina Schaffer
Disclosure: Obayashi reports receiving research grants from EnviroLife Research Institute Co., Sekisui Chemical Company, Tokyo Electric Power Company and YKK AP Inc.
Perspective
Back to TopPhyllis C. Zee, MD, PhD, and Ivy Cheung, PhD
Exposure to higher daytime and lower nighttime light intensity may help curb weight gain.
In a prospective (median follow-up of 21 months), population-based study of older Japanese adults, Obayashi and colleagues provide compelling evidence linking light exposure patterns to obesity risk. The method of light exposure in this study allowed for improved measurement of light intensity during the typical wake period, with a wrist light meter, while during the typical nocturnal sleep period, a portable light meter was also used. The findings demonstrate that after adjustment for confounders, such as caloric intake, physical activity level and sleep, evening or overnight light exposure of higher intensity and longer duration was associated with gains in waist-to-height ratio and BMI. Remarkably, increased risk for weight gain was seen with prolonged exposure to light intensities of just 100 lux or higher in the evening and nighttime exposure to light intensities of 3 lux or higher. On a positive note, increased daytime exposure to at least 500 lux and to overnight light exposure of lower intensities (< 3 lux) were associated with reductions in waist-to-height ratio and BMI.
These results highlight the potential of biologically appropriately timed light and dark interventions for promoting metabolic health. The relatively low levels of median mean morning (345.9 lux) and daytime (337.2 lux) light levels seen in this study suggest that the source of most of the light exposure in this older adult population was from indoor lighting. Although it would be ideal to increase daytime light and evening/nighttime darkness by exposure to the natural physical light and dark cycle, this is not feasible for most people. Thus, it may prove beneficial to explore artificial lighting paradigms adaptive to the time of day and night that are not detrimental, but actually favorable, to our health. This study focused on light intensity levels, but manipulation of the spectral properties of light represents an additional way to decrease the harmful effects of light at night and enhance the benefits of daytime light. For example, technologies that modify color displays in electronics to longer wavelength spectrums in the evening may allow for the needed visibility in older adults and safety at night. Additional investigation is warranted to determine generalizability to younger populations. Considering the pervasiveness of obesity and its related adverse health outcomes and the potential of light manipulations as a modifiable risk factor, research is needed to understand the mechanisms linking light and metabolism, as well as research to determine the impact of light-dark manipulations to improve metabolic health.
Disclosure: Cheung and Zee report no relevant financial disclosures.
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