Study finds bright nights raise risk for stroke and heart failure in adults over 40

Researchers have discovered that people exposed to brighter light at night face up to 50% higher risks of heart disease, while daytime light may protect the heart by reinforcing healthy circadian rhythms.

Study: Light Exposure at Night and Cardiovascular Disease Incidence. Image Credit: Krakenimages.com / Shutterstock

In a recent study published in JAMA Network Open, researchers explored whether being exposed to light at nighttime is associated with a higher risk of developing heart disease, particularly for those of a specific age, sex, or genetic makeup.

Their findings indicate that people over 40 exposed to bright lights at night face higher risks of heart disease, including stroke and heart failure. Associations were larger in females for heart failure and coronary artery disease, and in younger participants for heart failure and atrial fibrillation, with no clear modification for myocardial infarction or stroke.

Background

Healthy cardiovascular function relies on well-regulated circadian rhythms, which in turn influence vascular function, glucose tolerance, hormone levels, blood pressure, and heart rate. Disruption of these rhythms, through exposure to light or irregular sleep patterns, can elevate blood pressure and heart rate, increase inflammation, and reduce heart rate variability.

Animal studies show that prolonged circadian disruption can cause structural heart changes, such as hypertrophy and fibrosis. It worsens heart failure. Epidemiological evidence also links shift work, which disturbs these rhythms, to greater cardiovascular mortality, coronary heart disease, and heart failure.

Light exposure at night is a key source of circadian disruption and has been linked to higher rates of coronary artery disease and stroke, as well as conditions like obesity, diabetes, and hypertension, which are known cardiovascular risk factors. However, previous studies often relied on satellite-based measures of outdoor lighting or on small cohorts rather than on direct personal light-exposure data.

Using wrist-worn light sensors from about 89,000 UK Biobank participants, earlier research found that brighter nights were associated with higher cardiometabolic mortality and type 2 diabetes. Building on this, the present study examined whether individual day and night light exposures predict incident cardiovascular diseases over 9.5 years of follow-up.

About the Study

This large-scale cohort study used data from UK Biobank participants who wore wrist-worn light sensors for one week between 2013 and 2016. Participants’ light exposure was recorded continuously, processed to remove invalid data, and averaged into 24-hour profiles.

Factor analysis identified two main exposure periods: daytime (7:30 AM–8:30 PM) and nighttime (12:30 AM–6:00 AM). Participants were categorized into light-exposure percentiles, 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. Individuals with pre-existing cardiovascular disease (CVD) were excluded.

Cox proportional-hazards models assessed the relationship between light exposure and disease risk, adjusting sequentially for demographic factors (ethnicity, age, and sex), socioeconomic variables (deprivation, education, and income), and lifestyle factors (urbanicity, diet, alcohol, smoking, and physical activity). Additional models were tested for potential interactions with genetic risk scores, age, and sex.

Key Findings

Researchers analyzed data from 88,905 UK Biobank participants, with an average age of 62.4 years and 57% female, over an average follow-up of 7.9 years. Participants were free of cardiovascular disease at baseline.

Nighttime light exposure showed a clear, dose-dependent association with a higher risk of heart disease, while daytime light exposure was linked to lower risks in minimally and socioeconomically adjusted models, but these associations were not significant after full lifestyle adjustment. When physical activity was excluded from the full model, inverse associations re-emerged for heart failure and stroke.

Compared with those in the darkest-night environment, participants with the brightest night exposure had significantly greater risks of coronary artery disease, myocardial infarction, heart failure, atrial fibrillation, and stroke after adjusting for lifestyle, demographic, and socioeconomic factors.

In contrast, an increase in night-light exposure by one standard deviation raised the risk of all five cardiovascular outcomes by about 5–8%. The associations were consistent across models and remained robust after adjustments. Sex and age showed selective modifying effects, with larger associations in females for heart failure and coronary artery disease, and in younger individuals for heart failure and atrial fibrillation, with no clear modification for myocardial infarction or stroke. Associations also remained after accounting for polygenic risk, suggesting gene–environment correlation is unlikely to explain the results.

Conclusions

This large prospective study demonstrates strong associations of higher nighttime light exposure with elevated cardiovascular risk, though causality cannot be inferred. The mechanisms underlying this association could include circadian disruption and sleep disturbance, leading to vascular and metabolic stress. Reduced melatonin secretion was not directly examined in this study.

In contrast, greater daytime light exposure may support cardiovascular health by reinforcing 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 (primarily White participants), lack of information on light sources, and the 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 unavailable, limiting the ability to adjust for behaviors correlated with light exposure.

Overall, these findings highlight artificial nighttime lighting as a potentially modifiable environmental risk factor for cardiovascular disease, underscoring the importance of maintaining dark nights and adequate daylight exposure in urban health strategies.