An international cohort study of the effect on adults of exposure to light at night found that it significantly elevates the risk for cardiovascular disease (CVD) in those aged more than 50 years.

“In addition to current preventive measures, avoiding light at night may be a useful strategy for reducing risks of cardiovascular diseases,” wrote Daniel P. Windred, PhD, and his colleagues. Windred is affiliated with Flinders Health and Medical Research Institute in Bedford Park, South Australia.

For the study, 88,905 individuals with a mean age of 62.4 (SD, 7.8) years were included. Female participants were 56.9% of the study’s total population. The cohort was drawn from about 502,000 UK Biobank participants recruited between 2006 and 2010, 103,669 of whom wore light-tracking devices on their dominant wrist for 1 week between 2013 and 2016. CVD incidents were tracked through November 2022. 

In all, 13 million hours of light exposure data were captured from the wrist light sensors and stratified into the 0-50th, 51st-70th, 71st-90th, and 91st-100th percentiles. The investigators also analyzed over 700,000 person-years of follow-up.

The investigators found that, after adjusting for established CV risk factors (physical activity, smoking, alcohol, diet, sleep duration, preexisting diabetes, socioeconomic status, polygenic risk, ethnicity, shift work, etc), the cohort in the 91st-100th percentile, ie, those who experienced the brightest nights, had significantly higher risks of developing a range of CVDs than individuals in the 0-50th percentile group.

The bright night cohort’s risk for coronary artery disease was an adjusted hazard ratio (HR) of 1.32 (95% CI, 1.18-1.46). The adjusted HR for myocardial infarction was 1.47 (95% CI, 1.26-1.71), whereas for heart failure, it was 1.56 (95% CI, 1.34-1.81). Atrial fibrillation was found to have an adjusted HR of 1.32 (95% CI, 1.18-1.46). The adjusted HR for stroke in the bright light cohort was 1.28 (95% CI, 1.06-1.55).

“These findings are consistent with experiments demonstrating sleep-independent effects of light on circadian regulation of factors known to influence cardiovascular health, such as the secretion of glucagon-like peptide 1,” Windred and colleagues wrote.

The female cohort had larger-magnitude associations of night light with risks for heart failure (P for interaction = .006) and coronary artery disease (P for interaction = .02). In the youngest of the cohort, there were larger-magnitude associations of night light with risks for heart failure (P for interaction = .04) and atrial fibrillation (P for interaction = .02).

“These findings are consistent with previous research showing that exposure to shift work, which causes circadian disruption, predicts higher risk for heart failure in women compared with men,” Windred and colleagues wrote.

One novelty of this study is its investigation into whether light exposure is associated with heart failure or atrial fibrillation, the authors wrote. 

Previous studies of night light’s impact on human health have been based primarily on satellite-derived data, which cannot be relied upon to gauge light exposure at the individual level, making this in-person study novel.

A previous cohort study that used satellite data to define light exposure found that people with the brightest outdoor nights (top 20%) had a 7%-23% greater risk for coronary heart disease than those with the darkest outdoor nights (lowest 20%), wrote Windred and his co-authors. 

“We also observed a 28%-30% higher risk of stroke, whereas a previous satellite data study found a 26%-43% higher risk of stroke for people with the brightest outdoor nights (top 25%) compared with those with darker outdoor nights (lowest 25%),” the investigators said. “Furthermore, we observed a 45%-56% higher risk of heart failure and a 28%-32% higher risk of atrial fibrillation for people with the brightest nights.”

The individual-level data captured in this study also enabled the investigators to verify that the effects of night light exposure were independent of several factors that may go hand in hand with mistimed light exposure, Windred and coauthors said.

“Indeed, the associations of [night light] exposure with cardiovascular risk remained significant even in models adjusting for the participants’ preference in sleep timing (chronotype), actigraphy-based sleep duration, sleep efficiency, physical activity, and photoperiod,” wrote Jonathan Cedernaes, MD, PhD, in an accompanying editorial. “This indicates that [night light] exposure risks were not secondary to, for example, curtailed sleep, or seasonal day length.”

Cedernaes is a medical cell biology researcher in the Department of Medical Sciences at Uppsala University in Uppsala, Sweden.

Although he praised what he called the “unprecedented” work of Windred and colleagues, he said what remains to be discovered is whether this is an association between sleep timing and light exposure. 

“It therefore remains to be determined how light exposure can increase the risk of CVDs independent from, or in interaction with, changes in sleep timing,” Cedernaes wrote. 

The secretion of melatonin is one possible mechanism by which exposure to night light can adversely impact health, according to Cedernaes. “Light exposure that occurs around bedtime can delay and blunt melatonin secretion and, thus, our rhythms, including sleep-wake timing,” he wrote. 

Additional studies may be able to uncover the extent to which adults will be able to reduce their risk for CVD by decreasing their exposure to light at night over time, Cedernaes wrote, and if there is a critical life period when such interventions have the most significant effect on the underlying mechanisms for future CVD risk. 

“The impetus is now on us as researchers and healthcare practitioners to identify those at heightened risk from night light exposure, mitigate the hazards posed by unnecessarily bright artificial nighttime lighting, and optimize population-level 24-hour light exposure profiles,” he said. 

To that end, a family physician in Indiana told Medscape Medical News, “If I have a patient who works night shifts, I’ll often talk with them about how that schedule might be affecting their health.”

Juan Carlos Venis, MD, MPH, family physician and owner of INCircle Health Direct Primary Care in Indianapolis, said that discussing the potential adverse health risks of shift work with his patients is important. “Even if they can’t make changes right away, it opens the door for reflection about their future priorities. I’ve had patients who later chose to change shifts or jobs after noticing how much better they felt when their schedule matched their body’s natural rhythm. This is especially important for people who already have other risk factors for heart disease.”

Windred reported having no disclosures. Cedernaes reported receiving grants from the Swedish Society for Medical Research, Swedish Brain Foundation, and Swedish Research Council and speaker fees related to sleep research and treatment guidelines from Takeda Pharma and the publishing house Bokförlaget Stolpe outside the submitted work. Venis reported having no disclosures.