Circadian rhythms play a vital role in maintaining cardiovascular health by regulating heart rate, metabolic processes, endothelial function, hormone levels, and blood pressure. Short-term disruptions can lead to elevated heart rate, increased coagulation and inflammation, and decreased cardiac vagal tone. Chronic disruptions such as those caused by rotational shift work have been linked to a significantly elevated risk of cardiovascular disease (CVD). Among various environmental factors, light exposure at night is a key disruptor of the circadian rhythms. It has been recognized as a possible risk factor for CVD. Both sensors and satellite-based studies have associated brighter night environments with increased rates of CVD.
This study assesses how nighttime and daytime light exposure influence the occurrence of CVD, and how these associations vary by sex, age, and genetic predisposition. It used wrist-worn light sensors, and researchers found that darker days and brighter nights were associated with elevated cardiometabolic mortality and an increased rate of type 2 diabetes, which is a CVD risk factor.
This study included 103,669 participants from the United Kingdom (UK) Biobank who wore light-tracking machines for one week from 2013 to 2016. Cases of CVD were tracked till November 2022. The data was gathered by using personal light tracking machines and assessed by using factor analysis.
Diagnoses of CVD were collected from primary care, hospital admissions, death register records, and self-report as per ICD-10 and ICD-9 diagnostic criteria. Stroke and myocardial infarction were evaluated as per the UK Biobank’s algorithmically derived results.
The study focused on outcomes such as coronary artery disease, including myocardial infarction, coronary artery operations, and ischemic heart disease (IHD). Factors like gender, age, income, ethnicity, employment, education, material deprivation, alcohol consumption, urban location, healthy diet, smoking, and shift work status were collected from baseline questionnaires distributed from 2006 to 2010. Photoperiod was defined as the interval from sunrise to sunset. Weekly accelerometer recordings were used to determine average physical activity.
The risks of incident CVD were evaluated by using the Cox proportional hazards model with both night and daylight exposure as the key predictors. There are four percentile categories of night light and day light exposure for Cox models: 0 to 50th (reference group), 50 to 70th, 70 to 90th, and 90 to 100th.
Nighttime light exposure was associated with an increased risk of CVD in a clear, dose-dependent manner. Compared to individuals with the darkest nights (0 to 50th percentiles), those exposed to moderately brighter nights (50 to 70th percentiles) had a 1.11 to 1.12 times higher risk of coronary artery disease, those in the 70 to 90th percentile had a 1.18 to 1.20 times higher risk and those in the brightest category (90 to 100th percentile) had a 1.23 to 1.32 times higher risk. The risk of myocardial infarction rose with increasing night light exposure: 1.20 to 1.21 times higher in the 50 to 70th percentile, 1.27 times in the 70 to 90th, and 1.42 to 1.47 times in the brightest group. Heart failure risk increased by 1.15 times in the 50 to 70th percentile, 1.19 to 1.21 times in the 70 to 90th, and 1.45 to 1.56 times in the 90 to 100th percentile. Those in the highest night light exposure group also faced a 1.28 to 1.32 times greater risk of atrial fibrillation and a 1.28 to 1.30 times higher risk of stroke.
In contrast, greater exposure to daylight was associated with reduced risks of coronary artery disease, heart failure, and stroke. Participants in the brightest daylight group (90 to 100th percentiles) had a 11 to 13% lower risk of coronary artery disease (adjusted hazard ratios [aHR]: 0.87 to 0.89), a 23 to 28% lower risk of heart failure (aHR: 0.72 to 0.77), and a 25 to 27% lower risk of stroke (aHR: 0.73 to 0.75) compared to those with the darkest days (0 to 50th  percentiles). Although there were indications that brighter daylight might also lower the risks of myocardial infarction and atrial fibrillation, these associations did not reach statistical significance. Men had significantly higher risks for all cardiovascular outcomes compared to women, with adjusted HR ranging from 1.46 to 2.57, and risks increased with age. The strength of association between night light exposure and outcomes like coronary artery disease, atrial fibrillation, and heart failure varied notably by sex and age group.
Nighttime light exposure emerged as a significant risk factor for the development of CVD in this cohort. These findings suggest that, alongside existing prevention strategies, minimizing exposure to light during nighttime hours could be a beneficial approach to lowering cardiovascular risk.
Reference: Windred DP, Burns AC, Rutter MK, et al. Personal night light exposure predicts the incidence of cardiovascular diseases in >88,000 individuals. medRxiv. 2025. doi:10.1101/2025.06.20.25329961
