Nighttime light exposure may be linked with top killer disease worldwide

Sleeping in total darkness may lower the risk for cardiovascular disease, the top cause of death worldwide.

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(CNN) — Turning off the lights at night has long been known to support restful sleep, but new research suggests it may have another significant benefit. Sleeping in total darkness may lower the risk for cardiovascular disease, the top cause of death worldwide.

People who slept in the brightest conditions — similar to having overhead bedroom lights on —had a 56% greater risk of developing heart failure. Those who slept under such intense light also had a 32% higher risk of coronary artery disease and a 28% higher risk of stroke.

Wrist wearables tracked the study participants’ exposure to light intensity between 12:30 a.m. and 6 a.m., according to Dr. Daniel Windred, co-first author of the study published Thursday in the journal JAMA Network Open.

“One limitation is that we do not know the sources of personal light exposure, only the intensity,” said Windred, a research associate at Flinders University’s College of Medicine and Public Health in Australia. “Understanding typical sources of night light exposure might allow for more informed recommendations beyond ‘avoid bright light at night.’”

Light can block your brain’s production of the sleep-inducing hormone melatonin, so reducing light exposure at night can start with making changes to your wind-down routine. Limit screen time as much as possible and switch off unnecessary household lights within four hours of bedtime, said Dr. Julio Fernandez-Mendoza, a clinical psychologist and director of behavioral sleep medicine at the Penn State Health Sleep Research and Treatment Center, via email. He wasn’t involved in the study.

If turning the lights off isn’t possible, try to use dim or warm lighting instead of bright overhead lights, Windred said. In your bedroom, avoid using bright alarm clocks and sleeping near windows, if possible. Light-blocking window shades and blackout curtains can be helpful, as can a sleep mask over your eyes.

To the study authors’ knowledge, their work represents “the largest known study” of long-term associations between individual light exposure and cardiovascular disease risk, they wrote.

“We analysed 13 million hours of light data (for nearly) 89,000 people,” Windred said via email. The research builds upon previous studies that found similar associations and established disruption to circadian rhythms as a known risk factor for poor cardiovascular health.

Circadian rhythms are 24-hour cycles “in virtually every cell and tissue in our body, including our cardiovascular system” Windred said. That internal clock controls your sleep and wake cycles, detecting light as a signal to be awake and darkness as a signal to fall asleep.

Light’s possible impact on heart health

The study participants, about age 62 on average, were from the UK Biobank study, which has followed the health outcomes of more than 500,000 people ages 40 to 69 in the United Kingdom since 2006.

They wore light trackers for a week during their normal lives between 2013 and 2022 and then were monitored for certain health outcomes for nine years on average — one of the factors that made “the study unique and highly innovative,” Fernandez-Mendoza said. People who had cardiovascular issues before the tracking period were excluded.

Those exposed to the brightest light also had a 47% greater risk of having a heart attack and a 32% higher risk of developing atrial fibrillation. A-fib is an irregular, quivering or often rapid heart rhythm resulting from the heart’s upper chambers, the atria, beating out of sync with the lower chambers, the ventricles. Women faced greater odds for heart failure and coronary artery disease, while younger people were more affected by heart failure and A-fib.

How light may affect your circadian rhythm

The study, however, only shows a link and doesn’t prove light exposure causes cardiovascular issues. “There may be other differences in people exposed to light at night that are responsible for the greater risk of heart disease,” said Dr. Tim Chico, professor of cardiovascular medicine at the University of Sheffield in England, in a statement provided by the Science Media Centre. Chico wasn’t involved in the research.

The associations, however, were still significant after considering physical activity levels, smoking status, alcohol intake, diet, shift work and other potentially influential factors, the authors said. Short sleep duration affected only some of the findings, such as those for stroke.

The study is also “one of the few to present compelling evidence that high daytime light exposure, for example after rising in the morning, is linked to good cardiovascular health,” Fernandez-Mendoza said.

Light’s disruption of circadian rhythms has previously been found to dysregulate various cardiovascular and metabolic processes, such as damaging cells that help arteries function properly, causing high blood pressure and raising the risk for associated diseases, the authors said. Circadian disruption may also contribute to an increased tendency for blood to form clots, which can lead to strokes and heart attacks.

It’s also possible that the body responds to nighttime light “as a stressor, an unwanted event, and activates the body with high and abnormal heart rate, stress hormones, glucose and insulin, and inflammation,” Fernandez-Mendoza said.

Two of the authors are cofounders for Circadian Health Innovations, which makes a wrist-worn light sensor, but the company had no role in the study and isn’t the manufacturer of the tracker worn by study participants — nor is the other company with which a Circadian Health Innovations cofounder has a relevant pending patent without royalties in Australia.

Given the lack of participant diversity — 97% were White — and of details on sleep quality, as well as the brief light-tracking period, more studies are needed to understand how changes to lighting could reduce cardiovascular disease risk, the researchers wrote.

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