Accumulating evidence over the past decade suggests that light at night predicts increased risks of multiple adverse health outcomes. Why might this be so? Animal studies show that artificial light exposure at night affects melatonin signaling and disrupts sleep and circadian rhythms, potentially promoting weight gain and obesity. Would nighttime light similarly disrupt human circadian rhythms?
Light at night predicts increased body weight in women
Studies prior to 2019 of artificial light at night in relation to sleep have major limitations, because they mostly involve shift workers and typically use cross-sectional designs. Researchers in North Carolina used data from 43,722 women participants with an average age of 55 years in the Sister Study to investigate links between various types of artificial light at night and overweight or obesity. At baseline and after adjusting for confounding factors, having light or television in the bedroom predicted significantly higher prevalence of three markers of obesity: waist circumference of 34.6 inches or more (22 percent higher), waist-to-hip ratio of 0.85 or greater (13 percent higher), and waist-to-height ratio of 0.5 or more (15 percent higher). After an average follow-up of 5.7 years, compared to having no light or television in the bedroom, having light or television in the bedroom predicted significantly higher risks of gaining 11 lb of body weight (17 percent increase), increasing body mass index into the overweight category (22 percent higher), and increasing body mass index into the obesity category (33 percent higher). While advice to lose weight typically features increased physical activity and better diet, keeping the bedroom dark during sleep may also help.
Light at night may promote atherosclerosis
Using cross-sectional data from elderly Japanese residents in 2016, Japanese researchers found that higher intensity of bedroom light at night predicted increased carotid artery thickness, a measure of atherosclerosis burden. These researchers extended their earlier work 3 years later with longitudinal data from 989 participants with an average baseline age of 71 years in the HEIJO-KYO cohort. Sensors mounted on bedroom headboards measured night intensity over 2 nights. After adjusting for cardiovascular disease factors, participants in the highest quartile of bedroom light at night (9.3 lux) had significantly greater increases in average and maximum carotid artery thickness compared to participants in the lowest quartile (zero lux) at the end of 34 months of follow-up. By way of comparison, 3 lux is very dim light, too dark to read comfortably, while 10 lux is comparable to a candle flame one meter away. Thus, even relatively small amounts of light at night in the bedroom may promote carotid artery atherosclerosis.
Outdoor light at night may increase risk of coronary heart disease
Emerging epidemiological evidence suggests that outdoor light at night predicts increased risk of chronic diseases. Researchers in the US and in Hong Kong by Sun and others used satellite data to estimate nighttime outdoor light levels at 58,692 residences of individuals in Hong Kong to determine if higher levels of outdoor light at night predicted increased risk of coronary heart disease. After a median follow-up of 11 years and after adjusting for confounding factors and compared to participants exposed to the lowest quintile of outdoor light at night, participants exposed to the highest quintile had significant 23 and 29 percent increased risks of developing coronary heart disease and dying from coronary heart disease, respectively. Outdoor light at night might leak into bedrooms and disrupt circadian rhythms and adversely affect the function of coronary arteries.
An editorial in response to the article by Sun and others supported their conclusions about the harmful effects of light at night. The Sun study was among the first to prospectively examine links between exposure to light at night and coronary heart disease. Authors of the editorial reconfigured tabular data from the Sun study and more clearly showed the statistically significant trend in both hospitalizations and deaths from coronary heart disease in relation to progressively increasing levels of outdoor nocturnal light exposure. The editorial noted that the participants in the Sun study lived in Hong Kong, a densely-populated city with major air pollution, which limits the generalizability of the study.
Experimental evidence that bedroom light at night impairs cardiometabolic function
Exposure to artificial light at night while sleeping may adversely affect metabolic regulation. Researchers at Northwestern University conducted an experiment in which 20 healthy young adults were randomized to either a room light condition or a dim light condition for 2 nights. Dim light was 3 percent as bright at room light. The room light condition included one night of dim light during sleep followed by one night with room light during sleep. The dim light condition included two consecutive nights of dim light during sleep. Measures of insulin resistance were significantly higher in the room light compared to the dim light condition. In the room light condition, participants spent proportionately more time in stage N2 sleep and less time in slow wave and rapid eye movement sleep than participants in the dim light condition. Heart rate and heart rate variability were both significantly higher in the room light condition. In addition, blood insulin levels 30 minutes after administering glucose to participants were significantly higher in room light participants. These findings suggest that sleeping with regular room light may impair cardiometabolic function in young adults.
Light at night may dysregulate circadian clocks
Light at night commonly occurs in developed countries and is linked to adverse health. Researchers at Northwestern University used cross-sectional data from 552 older (average age 72 years) community-dwelling adults in a larger study to determine if objectively measured levels of light at night predicted higher odds of cardiovascular disease risk factors. Wrist-worn devices measured light levels during sleep. No light at night was defined as having no detectable light during the 5-hour period of lowest light exposure. After adjusting for confounding factors and compared to participants with no light at night, participants with light at night had significantly higher odds of having obesity (82 percent higher), diabetes (100 percent higher), and hypertension (74 percent higher). These higher odds may reflect dysregulation of circadian clocks that govern physiological processes, metabolic pathways, and the autonomic nervous system.
Outdoor light at night and air pollution
Chinese researchers used data from 28,302 participants with an average age of 62 years in the Yinzhou cohort to determine if outdoor light at night and air pollution predict risk of aspects of cerebrovascular disease (fatal and non-fatal stroke) over 4.5 years of follow-up. After adjusting for confounding factors and compared to participants in the lowest quartile of outdoor light at night exposure, participants in the highest quartile had a significant 43 percent higher risk of having a stroke. For air pollution, compared to participants who were exposed to the lowest quartiles of PM2.5 and PM10 particles, participants exposed to the highest quartile had significant 41 and 50 percent higher risk of a having stroke. For NO2 (nitrogen dioxide), compared to participants who were exposed to the lowest quartile of NO2, participants exposed to the highest quartile had significant 31 percent higher risk of a having stroke. Alas, the detrimental effects of outdoor light at night and NO2 seemed to be partially additive.
Indoor light at night and diabetes risk
Light at night may increase risk of chronic diseases, perhaps due to circadian disruption. Researchers in Australia and elsewhere used data from 84,790 participants with an average of 62 years in the UK Biobank to determine if indoor light at night measured at the individual person level predicted risk of developing diabetes during 7.9 years of follow-up. Wrist-worn devices recorded light exposure over 7-day periods. After adjusting for confounding factors and compared to participants with dark nights (in the 0-50th percentiles of light), participants exposed to more indoor light at night had progressively and significantly higher risks of developing diabetes during follow-up. Participants in the 90-100th percentiles of nighttime light (most nighttime light) had significant 53 percent higher risk of developing diabetes. Modeling suggested that diminished circadian amplitude and circadian phase that deviated from the group average predicted higher risk of diabetes. Finally, the risk of developing diabetes based on polygenic risk was comparable to the increase in risk from the lowest to the highest scores for light at night. Overall, reducing indoor light at night could be an effective way to reduce diabetes risk.
Bedroom light at night and cardiovascular disease
Light at night disrupts circadian rhythms, which are linked to changes in systolic and diastolic blood pressure, blood vessel function, and heart rate variability. Thus, light at night might increase risk of cardiovascular disease. To address this possibility, the above researchers in Australia and elsewhere used data from the UK Biobank for 88,905 participants with an average of 62 years who wore wrist mounted light sensors for a week. Light exposures at night were grouped in four categories: 0-50th, 51-70th, 71-90th, and 91-100th percentiles with higher percentiles reflecting greater nighttime bedroom light exposure. Compared to participants with dark nights (0-50th percentiles), participants with the brightest nights (91-100th percentiles) had significantly higher risks of developing coronary artery disease (32 percent), heart attack (47 percent), heart failure (56 percent), atrial fibrillation (32 percent), and stroke (28 percent) over 9.5 years of follow-up. Female and younger participants had even higher risks. The increased risks occurred independent of cardiovascular risk factors, such as low physical activity, smoking, and short sleep duration. Sleeping in a dark room may be a novel way to reduce our risk of cardiovascular diseases.
What to do
There may not be much that we as individuals can do in the near term about outdoor light at night. But right now we can reduce the amount of indoor light at night in our homes. For example, we can use an eye mask while sleeping, install dark curtains on our bedroom curtains, use a low-light nightlight, and move the TV out of the bedroom.
