Researchers found that people exposed to brighter light at night face up to a 50% higher risk of heart disease, while daylight may protect the heart by promoting healthy circadian rhythms.
Study: Exposure to light at night and incidence of cardiovascular disease. Image credit: Krakenimages.com / Shutterstock
In a recent study published in JAMA Network Openresearchers investigated whether exposure to light at night is associated with a higher risk of heart disease, particularly for people of a certain age, sex, or genetic make-up.
Their findings suggest that people over 40 who are exposed to bright light at night face higher risks of heart disease, including stroke and heart failure. Associations were greater in women for heart failure and coronary artery disease and in younger participants for heart failure and atrial fibrillation, without clear modification for myocardial infarction or stroke.
Background
Healthy cardiovascular function relies on well-regulated circadian rhythms, which in turn affect vascular function, glucose tolerance, hormone levels, blood pressure, and heart rate. Disrupting these rhythms, through exposure to light or irregular sleep patterns, can increase blood pressure and heart rate, increase inflammation, and decrease heart rate variability.
Animal studies show that prolonged circadian disruption can cause structural changes in the heart, such as hypertrophy and fibrosis. It worsens heart failure. Epidemiological evidence also links shift work, which disrupts these rhythms, to greater cardiovascular mortality, coronary heart disease, and heart failure.
Exposure to light at night is a major source of circadian disruption and has been linked to higher rates of coronary heart disease and stroke, as well as conditions such as obesity, diabetes and hypertension, which are known cardiovascular risk factors. However, previous studies often relied on satellite measurements of outdoor illumination or small cohorts rather than personal light exposure data.
Using wrist-worn light sensors from around 89,000 United Kingdom Biobanking participants, earlier research found that brighter nights were associated with higher cardiometabolic mortality and type 2 diabetes. Based on this, the present study examined whether individual exposure to daylight and nighttime light predicts cardiovascular disease events over 9.5 years of follow-up.
About the Study
This large-scale cohort study used data from United Kingdom Biobank participants wearing wrist-worn light sensors for one week between 2013 and 2016. Participants’ light exposure was continuously recorded, processed to remove invalid data, and averaged over 24-hour profiles.
Factor analysis identified two main exposure periods: daytime (7:30 AM–8:30 PM) and night (12:30 AM–6:00 AM). Participants were categorized into percentiles of light exposure, with the 0–50th percentile representing the darkest nights.
Cardiovascular outcomes, including stroke, atrial fibrillation, heart failure, myocardial infarction and coronary artery disease, were identified using hospital, primary care and death registry records. People with pre-existing cardiovascular disease (CVD) were excluded.
Cox proportional hazards models assessed the relationship between light exposure and disease risk, sequentially adjusting for demographic factors (ethnicity, age and sex), socioeconomic variables (deprivation, education and income) and lifestyle factors (urban area, diet, alcohol, smoking and physical activity). Additional models were tested for potential interactions with genetic risk scores, age, and sex.
Key findings
The researchers analyzed data from 88,905 United Kingdom Biobank participants, with a mean age of 62.4 years and 57% female, with a mean follow-up of 7.9 years. Participants were free of cardiovascular disease at baseline.
Nocturnal light exposure showed a clear, dose-dependent association with higher risk of heart disease, while daytime light exposure was associated with lower risks in minimally and socioeconomically adjusted models, but these associations were not significant after full adjustment for lifestyle. When physical activity was excluded from the full model, inverse associations for heart failure and stroke re-emerged.
Compared with those in the darkest nighttime environment, participants with the brightest nighttime exposure had a significantly higher risk of coronary heart disease, myocardial infarction, heart failure, atrial fibrillation, and stroke after adjusting for lifestyle, demographic, and socioeconomic factors.
Conversely, a one standard deviation increase in nighttime light exposure increased the risk of all five cardiovascular outcomes by approximately 5–8%. Associations were consistent across models and remained strong after adjustments. Gender and age showed selective modifying effects, with larger associations in women for heart failure and coronary artery disease and in younger subjects for heart failure and atrial fibrillation, with no clear modification for myocardial infarction or stroke. The associations also remained after accounting for polygenic risk, suggesting that gene-environment association is unlikely to explain the results.
conclusions
This large prospective study shows strong associations of higher nighttime light exposure with increased cardiovascular risk, although causality cannot be inferred. Mechanisms underlying this association could include circadian disruption and sleep disruption, leading to vascular and metabolic stress. Reduced melatonin secretion was not directly examined in this study.
Conversely, greater exposure to light during the day may support cardiovascular health by enhancing circadian rhythms.
Key strengths of this analysis include a large sample size, objective light measurements, and a long follow-up period. However, limitations include potential residual confounding, limited ethnic diversity (mainly White participants), lack of information about light sources, and inability to infer causality. Sleep duration and efficiency were objectively measured and included in sensitivity analyses. Short sleep partially attenuated some associations. Source information was not available, limiting the ability to adjust for behaviors related to light exposure.
Taken together, these findings highlight artificial night-time lighting as a potentially modifiable environmental risk factor for cardiovascular disease, underscoring the importance of preserving dark nights and adequate daylight exposure in urban health strategies.
