Where Bright Lights Are Switched on at Night, Heart Troubles Follow

The concept that there are cardiovascular harms of nighttime light exposure gained greater credence with a large new study.

Based on the U.K. Biobank cohort, people with more light exposure (>3 lux) during normal sleep hours typically showed worse cardiac structure and function on cardiac MRI scans 3 years later. Imaging findings showed left ventricular (LV) concentric hypertrophy and subclinical functional decline, impaired myocardial deformation, adverse changes in the right ventricle, and adverse changes in the left atrium in these individuals compared with peers having no nighttime light exposure, according to Lu Qi, MD, PhD, of Tulane University in New Orleans, and colleagues.

This ultimately translated into worse clinical outcomes over 8-10 years of follow-up. Participants with brighter nighttime light exposure had 29% higher risk of heart failure, 15% higher risk of atrial fibrillation, 24% higher risk of myocardial infarction, 33% higher risk of stroke, and 26% higher risk of cardiovascular disease (CVD) mortality, study authors reported in European Heart Journal.

“To our knowledge, this is the first study to characterize CMR [cardiac MRI]-derived cardiac phenotypes associated with nighttime light exposure. Our findings bridge the gap between prior studies linking nighttime light exposure to upstream CVD risk factors and those linking it to downstream clinical CVD endpoints, revealing adverse structural and functional alterations across multiple cardiac chambers and offering mechanistic insights into the light-CVD pathway,” they wrote.

Of note, Qi’s group found that the potential harms of nighttime light exposure seemed to be explained to some degree (24-49%) by shorter sleep duration.

Nevertheless, the effects attributed to light itself are biologically plausible. Light is, after all, the primary cue for entraining circadian clocks regulating blood pressure, heart rate, and myocardial metabolism, according to the authors.

It would appear from this study that the magnitude of effect of nighttime light exposure exceeds many of the residual risks targeted by current pharmacological prevention strategies, according to a trio led by Thomas Münzel, MD, of University Medical Center Mainz, Johannes Gutenberg University Mainz in Germany.

“It is time for clinicians, particularly those managing patients with heart failure or atrial fibrillation, to start asking about the sleep environment: how dark the bedroom is, whether the patient works night shifts, and how much screen use occurs after sunset,” they wrote in an accompanying editorial. “The advice is simple and inexpensive: blackout curtains, warm-colored bedside lighting, and covering the small standby LEDs on household electronics.”

“From a public health perspective, the implications are larger still. Light pollution is one of the few environmental hazards that cities can control directly and affordably. Shielded fixtures, warm-spectrum bulbs, and dimming or motion-activated streetlights are already available, energy-efficient, and climate-friendly,” the group noted.

Future mechanistic studies should pair light dosimetry with real-time measurements of melatonin, heart rate variability, ambulatory blood pressure, and markers of oxidative stress and inflammation, according to the editorialists.

For their population-based cohort study, Qi and colleagues included 11,071 U.K. Biobank participants with light sensor data and subsequent cardiac MRI scans. Mean age was 61.1 years; the cohort was 56.6% women and 97.6% white.

Participants had worn an Axivity AX3 triaxial accelerometer, featuring a built-in light sensor, for 7 consecutive days during a baseline period in 2013-2015. Nighttime was defined by each participant’s least active 5-hour period as a way to account for interindividual variability in circadian timing.

Nighttime exposure to light >3 lux in brightness was consistently associated with MRI findings even with multivariable adjustment:

• LV concentric hypertrophy and subclinical functional decline: 2.4% greater LV mass indexed to height, 1.5% greater mean wall thickness, and 1.9% lower myocardial contraction fraction
• Impaired myocardial deformation: lower absolute global LV circumferential, radial, and longitudinal strain indices
• Adverse changes in the right ventricle: 0.9% greater end-diastolic and 1.6% greater end-systolic volumes indexed
• Adverse changes in the left atrium: 2.3% greater maximum volume indexed and 1.1% lower ejection fraction

In contrast, daytime light exposure as bright as >1,000 lux had no bearing on subclinical findings and CVD outcomes.

Qi and colleagues acknowledged that the observational study could not establish causality, however, and their definition of light exposure was limited to a brief 1-week assessment period.

Among the study’s other limitations, the study devices did not account for interindividual variability in light sensitivity or the blue-heavy LED light now ubiquitous through consumer electronics. Shift workers and other vulnerable groups also remain understudied, having been excluded.